Title of Invention: WOUND CLEANSING APPARATUS WITH STRESS Abstract: An apparatus for cleansing wounds with means for stressing the wound bed and optionally tissue surrounding the wound, in which irrigant fluid from a reservoir connected to a conformable wound dressing and wound exudate from the dressing are recirculated by a devise for moving fluid through a flow path which passes through the dressing and a means for fluid cleansing and back to the dressing. The cleansing means (which may be a single-phase, e.g. micro-filtration, system or a two-phase, e.g. dialytic system) removes materials deleterious to wound healing, and the cleansed fluid, still containing materials that are beneficial in promoting wound healing, is returned to the wound bed. The means for stressing the wound bed and optionally tissue surrounding the wound promotes wound healing. The dressing and a method of treatment using the apparatus. Detailed Description: CROSS REFERENCE TO RELATED APPLICATIONS This application is a continuation of U.S. application Ser. No. 12/832,002, filed on Jul. 7, 2010, which is a divisional application of U.S. application Ser. No. 11/957,860, filed on Dec. 17, 2007, now U.S. Pat. No. 7,753,894, which is a continuation application of U.S. application Ser. No. 10/599,720, filed Oct. 6, 2006, which is the U.S. national phase application of PCT/GB2005/001577, filed Apr. 26, 2005, which claims priority to GB0409291.2, filed on Apr. 27, 2004, all of which are hereby incorporated by reference. BACKGROUND 1. Field The present invention relates to apparatus and a medical wound dressing for irrigating, stressing and/or cleansing wounds, and a method of treating wounds using such apparatus for irrigating, stressing and/or cleansing wounds. It relates in particular to such an apparatus, wound dressing and method that can be easily applied to a wide variety of, but in particular chronic, wounds, to cleanse them of materials that are deleterious to wound healing, whilst retaining materials that are beneficial in some therapeutic aspect, in particular to wound healing. 2. Background Before the present invention, aspirating and/or irrigating wounds and apparatus therefor were known, and tended to be used to remove wound exudate during wound therapy. In known forms of such wound therapy, the offtake from the wound, especially when in a highly exuding state, is voided to waste, e.g. to a collection bag. Materials deleterious to wound healing are removed in this way. However, materials that are beneficial in promoting wound healing, such as growth factors, naturally occurring anti-inflammatories, and other physiologically active components of the exudate from a wound are lost to the site where they can be potentially of most benefit, i.e. the wound bed, when such therapy is applied. It thus would be desirable to provide a system of therapy which a) can remove materials deleterious to wound healing from wound exudate, whilst retaining materials that are beneficial in promoting wound healing in contact with the wound bed, and/or b) which allows fluids containing active amounts of materials that are beneficial in promoting wound healing to pass into and/or through the wound in contact with the wound bed. Dialysis is a known method of treating bodily fluids such as blood ex vivo, to cleanse them of materials that are deleterious to the body systemically. Retaining materials that are beneficial in some therapeutic aspect in the treated fluid is not an object of dialysis. This method of treating bodily fluids is also a systemic therapy, since the treated fluid is returned to within the body. This is in contrast to a topical therapy in which the treated fluid is recycled outside the body, e.g. to a wound. Dialysis also requires large amounts either of bodily fluids, such as blood, or of dialysate, and consequently the relevant devices tend not to be portable. Even when in a highly exuding state, chronic wounds produce relatively little fluid to be treated and relatively little materials that are beneficial in some therapeutic aspect to be retained in the wound and/or its environment. SUMMARY It is an object of the present invention a) to obviate at least some of the abovementioned disadvantages of known aspiration and/or irrigation therapy systems, and b) to provide a system of therapy which i) can remove materials deleterious to wound healing from wound exudate, whilst retaining materials that are beneficial in promoting wound healing in contact with the wound bed, and/or ii) which allows fluids containing active amounts of materials that are beneficial in promoting wound healing to pass into and/or through the wound in contact with the wound bed. It is a further object of the present invention a) to obviate at least some of the abovementioned disadvantages of known dialysis systems, and b) to provide a system of therapy which i) can remove materials deleterious to wound healing from wound exudate, whilst retaining materials that are beneficial in promoting wound healing in contact with the wound bed, and ii) further allows fluids containing active amounts of materials that are beneficial in promoting wound to pass into and/or through the wound in contact with the wound bed, without affecting the body systemically. It is a yet further object of the present invention a) to obviate at least some of the abovementioned disadvantages of known dialysis systems, and b) to provide a system of therapy which i) can remove materials deleterious to wound healing from wound exudate, whilst retaining materials that are beneficial in promoting wound healing in contact with the wound bed, and ii) further allows fluids containing active amounts of materials that are beneficial in promoting wound to pass into and/or through the wound in contact with the wound bed without affecting the body systemically, and iii) is portable. Additionally, known forms of wound dressing and aspiration and/or irrigation therapy systems often create a wound environment under the backing layer that may result in the loss of optimum performance of the body's own tissue healing processes, and slow healing and/or in weak new tissue growth that does not have a strong three-dimensional structure adhering well to and growing from the wound bed. This is a significant disadvantage, in particular in chronic wounds. It is an object of the present invention a) to obviate at least some of the abovementioned disadvantages of known wound dressing, and b) to provide a system of therapy which i) can remove materials deleterious to wound healing from wound exudate, whilst retaining materials that are beneficial in promoting wound healing in contact with the wound bed, and ii) which creates stress across the wound bed and optionally tissue surrounding the wound, e.g. by applying an optionally varying positive and/or negative pressure to the wound. Such a stress across the wound bed and optionally tissue surrounding the wound, e.g. an optionally varying positive and/or negative pressure applied to the wound, has been found to result in an increase in improvements to wound healing, such as an increase in cell proliferation, revascularisation, improved breaking strength and reduction of wound recurrence. The resultant tissue growth has a strong three-dimensional structure adhering well to and growing from the wound bed. It also stimulates blood flow in underlying tissue and optionally tissue surrounding the wound. Removal of fluid by optionally varying negative pressure leads to reduction of interstitial oedema and pressure directly affecting the lymphatic and capillary system, restoring lymph function. All of these are beneficial to wound healing. Thus, according to a first aspect of the present invention there is provided an apparatus for irrigating, stressing and/or cleansing wounds, characterised in that it comprises a) a fluid flow path, comprising i) a conformable wound dressing, having a backing layer which is capable of forming a relatively fluid-tight seal or closure over a wound and at least one inlet pipe for connection to a fluid supply tube, which passes through and/or under the wound-facing face, and and at least one outlet pipe for connection to a fluid offtake tube, which passes through and/or under the wound-facing face, the point at which the or each inlet pipe and the or each outlet pipe passes through and/or under the wound-facing face forming a relatively fluid-tight seal or closure over the wound, at least one inlet pipe being connected to a fluid recirculation tube, and at least one outlet pipe being connected to a fluid offtake tube: and ii) a means for fluid cleansing having at least one inlet port connected to a fluid offtake tube and at least one outlet port connected to a fluid recirculation tube; b) a fluid reservoir connected by a fluid supply tube to an integer of the flow path (optionally or as necessary via means for flow switching between supply and recirculation); c) a device for moving fluid through the wound dressing and means for fluid cleansing, and optionally or as necessary the fluid supply tube; and d) optionally means for bleeding the flowpath, characterised in that it comprises e) means for stressing the wound bed and optionally tissue surrounding the wound; such that fluid may be supplied to fill the flowpath from the fluid reservoir via the fluid supply tube (optionally or as necessary via the means for flow switching) and recirculated by the device through the flow path. Where any pipe is described in connection with the apparatus as being connected or for connection to a (mating end of a) tube, e.g. a fluid supply tube, fluid recirculation tube or fluid offtake tube, the pipe and the tube may form a single integer in the flow path through which the circulating fluid from the wound passes. As noted hereinbefore, the present invention in this aspect advantageously provides a means for combining more than one therapy in a single dressing system, such as a) removal of materials deleterious to wound healing from wound exudate, whilst retaining materials that are beneficial in promoting wound healing, and b) promoting wound healing, by stimulating new tissue growth adhering well to and growing from the wound bed, by creating stress across the wound bed and optionally tissue surrounding the wound. It may aid in the debridement of slough, eschar and necrotic tissue growth from the wound. In all relevant embodiments of the present apparatus for irrigating, stressing and/or cleansing wounds, it is advantageous that it additionally, where appropriate, comprises a system which can regulate the pressure on the wound bed, under the wound dressing. Preferably such a system is a conventional automated, programmable system which can maintain the wound at or near an appropriate, desired stress to the wound bed and optionally tissue surrounding the wound, to an appropriate, desired programme while moving fluid over the wound bed and through the means for fluid cleansing at an appropriate, desired rate. Examples of suitable means for the stimulation of the healing of wounds and tissue adhering well to and growing from the wound bed include applying mechanical stimulus to the wound bed and optionally tissue surrounding the wound via the wound dressing and/or via the fluid under dressing. Examples of suitable ways in which this can in turn be achieved include applying an optionally varying positive and/or negative pressure at any appropriate point for stressing the wound. The amplitude of the positive and/or negative pressure on the wound bed and optionally tissue surrounding the wound and/or the fluid thereover may be constant, but more usually is varied, preferably cyclically, either randomly or regularly. Such cyclical variation of the pressure applied to the wound is effectively the application of an amplitude waveform at a desired frequency to apply a desired level of stress to the wound bed and optionally tissue surrounding the wound. The desired level and regime of stress to the wound bed and optionally tissue surrounding the wound may be applied conventionally by varying the positive and/or negative pressure applied to the wound bed, e.g. by a) varying the rate of the means for moving fluid over the wound bed and through the means for fluid cleansing, as appropriate or desired, e.g. the rate of any pump used to apply positive or negative pressure to the wound bed at any appropriate point for stressing the wound, b) bleeding fluids, especially gases, such as air and nitrogen, but not excluding liquids, such as water and saline, or gas in liquid aerosols; and gels into the flowpath in any appropriate part of the apparatus to vary the pressure applied to the wound to a desired level and/or programme, and/or c) varying the pressure in any inflatable filler within the wound dressing as appropriate or desired, as described in more detail hereinafter. Preferably, such regular or random variation of the positive and/or negative pressure applied to the wound will be effected by conventional process control devices and/or software. The stimulation of the healing of wounds in the present invention may also be effected by agitation of the wound bed and/or creating intermittent flow and/or turbulence to stimulate the cells. This can be done preferably by regularly or randomly pulsing a positive and/or negative pressure applied to the wound at any appropriate point for this purpose. Such pulsed variation of the pressure applied to the wound is again effectively the application of an amplitude waveform at a desired frequency to apply a desired level of stress to the wound bed and optionally tissue surrounding the wound. Pulsing the pressure on the wound may advantageously also provide a means to over-ride pain, similar to TENS. The range of variation of the pulsed positive and/or negative pressure applied to the wound will be substantially less than the maximum levels of pressure referred to below. The frequencies of such pulsed stressing across the wound will be substantially higher than those of the cycles of positive and/or negative pressure to the wound bed and optionally tissue surrounding the wound for the stimulation of the healing of wounds referred to above. The range of variation of the pulsed positive and/or negative pressure applied to the wound will be substantially less than the maximum levels of pressure and than the range of variation in the levels of pressure referred to below. The frequencies of such pulsed stressing across the wound will usually be substantially higher than those of the cycles of positive and/or negative pressure to the wound bed and optionally tissue surrounding the wound for the stimulation of the healing of wounds referred to above. Regularly or randomly pulsing any pressure applied to the wound may be effected essentially as described hereinbefore in connection with the variation of the positive and/or negative pressure applied to the wound. Again, such regular or random pulsing of the positive and/or negative pressure applied to the wound may be effected by conventional process control devices and/or software. Such pulsing of any pressure applied to the wound may be applied as an amplitude modulation of the positive and/or negative pressure applied to the wound bed and optionally tissue surrounding the wound, which in turn may be held constant, but more usually is varied, preferably cyclically, either randomly or regularly. Where the levels of such pressure above or below, atmospheric are held constant, this is often achieved in the present apparatus, where appropriate, by use of a control device that can regulate the pressure in the wound dressing by bleeding fluids, especially gases, such as air and nitrogen, but not excluding liquids, such as water and saline, or gas in liquid aerosols; and gels into the flowpath in any appropriate part of the apparatus to vary the pressure applied to the wound to a desired level and/or programme. This often results in any device for moving fluid through the wound that is downstream of the dressing and that applies an overall negative pressure in the wound space, e.g. a vacuum pump, pumping a heterogeneous mixture of liquid wound exudate and irrigant from the wound dressing with bleed gases, such as air and nitrogen. This can result in pulsing of any pressure applied to the wound. The pumping rate, the rate of bleeding, and the dimensions of the offtake and/or recirculation tubes may be adjusted to maintain the desired balance of pulsing pressure amplitude and frequencies on the wound. Preferably such a system is a conventional automated, programmable system which can maintain the appropriate pulse regimen to the wound. Stimulus to the wound bed and optionally tissue surrounding the wound by applying an optionally varying positive and/or negative pressure and agitation of the wound bed to stimulate the cells by regularly or randomly pulsing any pressure applied to the wound are mutually compatible. They may, as appropriate, be applied alone or together. Thus, an embodiment of the apparatus for irrigating, stressing and/or cleansing wounds of the present invention is characterised in that it comprises means for supplying optionally varying positive and/or negative pressure, which is optionally pulsed, to a wound bed and optionally tissue surrounding the wound for the stimulation of the healing of the wound. As noted hereinbefore, in the present invention in this aspect, the positive and/or negative pressure on the wound bed and/or the fluid thereover and optionally tissue surrounding the wound may be constant, but more usually is varied, preferably cyclically, either randomly or regularly. Where the pressure on the wound bed and/or the fluid thereover and optionally tissue surrounding is varied, it may be a varying positive or negative pressure, or it may as appropriate vary from positive to negative or vice versa, again preferably cyclically, and either randomly or regularly. It may vary about a constant positive or negative baseline pressure, or less usually about a varying baseline pressure. Examples of maximum levels of such pressure above and below atmospheric include 50% atm. e.g. between 5 and 40% atm., e.g. between 15 and 35% atm. Examples of suitable frequencies of such regular cycles of pressure for the stimulation of the healing of wounds include 1 to 48 per 24 hr, such as 12 to 24 per 24 hr, e.g. 2 to 1 per hr. Examples of suitable waveforms of such cycles either regularly or randomly for the stimulation of the healing of wounds include curved, e.g. sinusoidal, random white noise and sawtooth for higher frequencies, and usually square for lower frequencies. Examples of suitable frequencies of regular pulses for the stimulation of the healing of wounds include 1 to 3000 per min (0.016-50 Hz), e.g. 30 to 60 per min, e.g. 3 to 20 per min, i.e. 0.05 to 0.33 Hz, such as 5 to 10 per min. Such pulses may be varying positive or negative pressure pulses, or they may as appropriate vary from positive to negative or vice versa, again preferably cyclically, and either randomly or regularly. They may vary about a constant positive or negative baseline pressure or about a varying baseline pressure. Examples of maximum amplitudes for such pulses are up to 10 mm Hg above and below the constant positive or negative baseline pressure, e.g. up to 7 mm Hg or up to 3 mm Hg. Examples of suitable waveforms of such pulses either regularly or randomly for the stimulation of the healing of wounds include curved, e.g. sinusoidal, random white noise sawtooth, square and a systolic-diastolic asymmetric sawtooth. Where the amplitude of regular cycles of pressure is modulated by superimposed regular pulses for the stimulation of the healing of wounds, examples of suitable frequencies of the combination include those where the carrier frequency is 1 to 48 per 24 hours and the superimposed frequency of the pressure pulses is 1 to 0.05 Hz, both with the respective amplitudes noted above. Examples of suitable waveforms of the cycles and the superimposed pulses may be regular or random and include curved, e.g. sinusoidal, random white noise sawtooth, square and a systolic-diastolic asymmetric sawtooth. Examples of means for applying an optionally varying positive and/or negative pressure at any appropriate point for stressing the wound and/or regularly or randomly pulsing any pressure applied to the wound for promoting wound healing, whether applied alone or together, include a wound dressing as hereinbefore defined that comprises one or more expandable and contractible modules capable of applying pressure to the wound bed and optionally tissue surrounding the wound at any appropriate point for stressing the wound. Examples of other suitable means of applying mechanical stimulus to the wound by optionally varying positive and/or negative pressure include a magnetic fluid in a chamber or other hollow structure under the backing layer of the dressing in contact with the wound bed and/or the fluid thereover to which a regularly or randomly (preferably cyclically) varying and/or pulsing external magnetic field is applied. However, such means are less favoured. Thus, one favoured embodiment of the apparatus for irrigating, stressing and/or cleansing wounds is characterised in that it comprises a wound dressing as hereinbefore defined that comprises one or more expandable and contractible modules. These are capable of applying pressure to the wound bed and optionally tissue surrounding the wound at any appropriate point for stressing the wound. This means should be capable of maintaining the pressure on the wound bed and/or the fluid thereover and optionally tissue surrounding the wound at a constant level, but more usually it should also be capable of regularly or randomly (preferably cyclically) varying and/or pulsing the pressure applied to the wound, all at or near an appropriate, desired level of stress to the wound bed and optionally tissue surrounding the wound, to an appropriate, desired programme while moving fluid over the wound bed and through the means for fluid cleansing at an appropriate, desired rate. Examples of suitable modules capable of applying pressure to the wound bed at any appropriate point for stressing the wound include a module in the wound dressing may be made of a polymer that can be electrically stimulated to change shape repeatedly as appropriate frequencies. A preferred module is a fluid-inflatable body that lies in the wound in use. Thus, one favoured embodiment of the apparatus for irrigating, stressing and/or cleansing wounds is characterised in that it comprises a wound dressing as hereinbefore defined that comprises one or more fluid-inflatable modules capable of applying pressure to the wound bed at any appropriate point for stressing the wound. This is capable of maintaining the pressure on the wound bed and/or the fluid thereover at a constant level, but more usually it is also capable of regularly or randomly (preferably cyclically) varying and/or pulsing the pressure applied to the wound, all at or near an appropriate, desired level of stress to the wound bed, to an appropriate, desired programme while moving fluid over the wound bed. The module or modules is/are (usually cyclically) inflated and deflated by admitting and releasing fluid. Once the inflatable body has been inflated, it may be deflated as appropriate or desired, and then reinflated to again apply a positive pressure to the wound, and the cycle may be repeated as appropriate or desired. Alternatively, it may be partially filled with an elastically resilient material, such as an elastomeric foam, that in its rest state is capable of applying a working pressure to the wound bed. The body may then be deflated as appropriate or desired, and then reinflated under the action of its filler material to again apply a positive pressure to the wound, and the cycle may be repeated as appropriate or desired. Examples of forms of the body that are suitable such expandable and contractible modules capable of applying pressure to the wound bed at any appropriate point for stressing the wound include fluid-inflatable fillers and fluid-inflatable irrigant inlet manifolds comprised in the dressing as described hereinafter in greater detail. Where the module is a fluid-inflatable filler, examples of suitable fluids include gases, such as air and nitrogen; liquids, such as water and saline; gas in liquid aerosols; and gels such as those described in greater detail hereinafter. Preferred fluids include gases, such as air or nitrogen. Where the module is a fluid-inflatable irrigant inlet manifold comprised in the dressing as described hereinafter in greater detail, it will be stimulated to change shape as appropriate and optionally at desired frequencies by inflation with irrigant, followed as desired by deflation. Examples of both are included hereinafter. Examples of such fillers include a substantially flat film, sheet or membrane, defining a chamber, pouch or other structure of the backing layer, e.g. of polymer film, which can contain the fluid. It is provided with an inflation device for moving inflation fluid to the filler, and is connected to it by an inflation tube which communicates with its internal space. The inflation device may also serve as a deflation device for moving inflation fluid from the filler, and the inflation tube also serves as a deflation tube. Alternatively or additionally, where appropriate the filler as hereinbefore defined may have a deflation pipe and a bleed valve to waste, e.g. to a collection bag if a non-gaseous fluid is used. The inflation device for moving then only serves as an inflation device to apply a positive pressure on the wound bed. Less usually, the filler may have an inflation pump and a deflation pump. Where it lies under the backing layer of the wound dressing of the apparatus of the invention, the inflation tube may run to the filler within the wound under the wound-facing face of the wound dressing. However, the inflation tube may be connected to an inflation pipe that passes through the wound-facing face of the backing layer, the point at which the inflation pipe passes through the wound-facing face forming a relatively fluid-tight seal. The inflation pipe may be in the form of an aperture, such as a funnel hole, opening, orifice, luer, slot or port for connection as a female member respectively to a mating end of the inflation tube (optionally or as necessary via means for forming a tube, pipe or hose, or nozzle). Where the pipe passes through, rather than under the backing layer, the backing layer may often have a rigid and/or resiliently inflexible or stiff area to resist any substantial play between the or each pipe and the or each mating tube, or deformation under pressure in any direction. It may often be stiffened, reinforced or otherwise strengthened by a boss projecting distally (outwardly from the wound) around each relevant tube, pipe or hose, or nozzle, hole, opening, orifice, luer, slot or port for connection to a mating end of the inflation tube. The components may be a push, snap or twist-lock fit with each other. The minimum calibre of the inflation tube and pipe must be sufficient for them to permit as rapid inflation and deflation of the filler as is desired. Suitably the range of cross-dimensions of the bore may be 0.5 to 6 mm, e.g. 1.5 to 2 mm. Each should be resiliently flexible, and preferably soft with good conformability. This can be done for example by forming it of a suitable material, e.g. a resilient thermoplastic. Examples of suitable inflation device for moving fluid into the filler include pumps. As noted above, the inflation device may also serve as a deflation device for moving inflation fluid from the filler, and the inflation tube also serves as a deflation tube. In such case, the pump must be a reversible pump used to increase and decrease the pressure on the wound bed as desired. Subject to this consideration, the type and/or capacity of the device will also be largely determined by the appropriate or desired positive or negative pressure to the wound bed, the nature of the fluid, i.e. whether it is a gas, such as air and nitrogen; a liquid, such as water and saline; a gas in liquid aerosol; or a gel; the desired frequencies and waveforms of such cycles either regularly or randomly. The following types of pump may be used to apply positive pressure, as desired to the filler through an inflation tube which communicates with its internal space: reciprocating pumps, such as: syringe or piston pumps—providing high pressure and high accuracy; diaphragm pumps—where pulsations of one or two flexible diaphragms displace liquid while check valves control the direction of the fluid flow e.g. preferably a small portable diaphragm pump. and rotary pumps, such as: centrifugal pumps—with rotating vaned disk attached to a drive shaft moving fluid without pulsation as it spins. The outlet can be restricted without damaging the pump. peristaltic pumps—with rollers on a rotor acting on fluid in a tube, e.g. preferably a small portable peristaltic pump. Of these, only piston pumps and rotary pumps, such as centrifugal pumps and peristaltic pumps are readily reversible pumps that may be used to increase and decrease the pressure on the wound bed as desired. Subject to this consideration, preferred reversible pumps include a small portable peristaltic pump. Preferred non-reversible pumps to be used with a bleed valve to the filler then include a small portable syringe pump (which is often used once and then disposed of) or small portable diaphragm pump, e.g. a sphygmometric pump. As noted hereinbefore, stimulation of the healing of wounds in the present invention may also be effected by regularly or randomly pulsing a pressure applied to the wound at any appropriate point for this purpose. Such pulsed pressure on the wound may be provided by some types of the device for moving fluid into the filler. Where the module is a fluid-inflatable irrigant inlet manifold comprised in the dressing as described herein after in greater detail, it will be stimulated to change shape as appropriate and optionally at desired frequencies by inflation with irrigant, followed as desired by partial deflation. The device for moving fluid through the wound and means for fluid cleansing is used to move irrigant to inflate the inlet manifold and apply a positive pressure to the wound bed. As noted hereinafter, the device may suitably be a pump. As noted hereinbefore, the pressure on the wound bed may be constant, but may be varied, preferably cyclically, either randomly or regularly. To achieve this, the present apparatus, where appropriate, comprises a system that can regulate the pump output to the inlet manifold in the wound dressing. Preferably such a system is a conventional automated, programmable system which can maintain the wound at or near an appropriate, desired flow stress to the wound bed to an appropriate, desired programme while moving fluid over the wound bed. As noted hereinbefore, stimulation of the healing of wounds in the present invention may also be effected by regularly or randomly pulsing a pressure applied to the wound at any appropriate point for this purpose. Such pulsed flow across the wound may be provided by some types of the device for moving fluid through the wound and means for fluid cleansing. Certain diaphragm pumps described hereinafter in greater detail will be appropriate for this purpose, as are certain peristaltic pumps, and an electromechanical oscillator directly coupled to the wound dressing, will be suitable. Suitable materials for such modules of any type include synthetic polymeric materials that do not absorb aqueous fluids, such as polyolefins, polysiloxanes and polyesters. They may be hydrophilic, and thus also include hydrophilic polyurethanes. They also include thermoplastic elastomers and elastomer blends, for example copolymers, such as ethyl vinyl acetate polystyrene and elastomeric polyurethane formed by solution casting. In all embodiments of the apparatus of this first aspect of the present invention for aspirating, irrigating and/or cleansing wounds, a particular advantage is the tendency of the wound dressing to conform to the shape of the bodily part to which it is applied. The wound dressing comprises a backing layer with a wound-facing face which is capable of forming a relatively fluid-tight seal or closure over a wound and at least one inlet pipe for connection to a fluid supply tube or recirculation tube, which passes through and/or under the wound-facing face, and at least one outlet pipe for connection to a fluid offtake tube, which passes through and/or under the wound-facing face, the point at which the or each inlet pipe and the or each outlet pipe passes through and/or under the wound-facing face forming a relatively fluid-tight seal or closure. The term ‘relatively fluid-tight seal or closure’ is used herein to indicate one which is fluid-and microbe-impermeable and permits a positive or negative pressure of up to 50% atm., more usually up to 15% atm. to be applied to the wound. The term ‘fluid’ is used herein to include gels, e.g. thick exudate, liquids, e.g. water, and gases, such as air, nitrogen, etc. The shape of the backing layer that is applied may be any that is appropriate to aspirating, irrigating and/or cleansing the wound across the area of the wound. Examples of such include a substantially flat film, sheet or membrane, or a bag, chamber, pouch or other structure of the backing layer, e.g. of polymer film, which can contain the fluid. The backing layer may be a film, sheet or membrane, often with a (generally uniform) thickness of up to 100 micron, preferably up to 50 micron, more preferably up to 25 micron, and of 10 micron minimum thickness. Its largest cross-dimension may be up to 500 mm (for example for large torso wounds), up to 100 mm (for example for axillary and inguinal wounds), and up to 200 mm for limb wounds (for example for chronic wounds, such as venous leg ulcers and diabetic foot ulcers. Desirably the dressing is resiliently deformable, since this may result in increased patient comfort, and lessen the risk of inflammation of a wound. Suitable materials for it include synthetic polymeric materials that do not absorb aqueous fluids, such as polyolefins, such as polyethylene e.g. high-density polyethylene, polypropylene, copolymers thereof, for example with vinyl acetate and polyvinyl alcohol, and mixtures thereof; polysiloxanes; polyesters, such as polycarbonates; polyamides, e.g. 6-6 and 6-10, and hydrophobic polyurethanes. They may be hydrophilic, and thus also include hydrophilic polyurethanes. They also include thermoplastic elastomers and elastomer blends, for example copolymers, such as ethyl vinyl acetate, optionally or as necessary blended with high-impact polystyrene. They further include elastomeric polyurethane, particularly polyurethane formed by solution casting. Preferred materials for the present wound dressing include thermoplastic elastomers and curable systems. The backing layer is capable of forming a relatively fluid-tight seal or closure over the wound and/or around the inlet and outlet pipe(s). However, in particular around the periphery of the wound dressing, outside the relatively fluid-tight seal, it is preferably of a material that has a high moisture vapour permeability, to prevent maceration of the skin around the wound. It may also be a switchable material that has a higher moisture vapour permeability when in contact with liquids, e.g. water, blood or wound exudate. This may, e.g. be a material that is used in Smith & Nephew's Allevyn™, IV3000™ and OpSite™ dressings. The periphery of the wound-facing face of the backing layer may bear an adhesive film, for example, to attach it to the skin around the wound. This may, e.g. be a pressure-sensitive adhesive, if that is sufficient to hold the wound dressing in place in a fluid-tight seal around the periphery of the wound-facing face of the wound dressing. Alternatively or additionally, where appropriate a light switchable adhesive could be used to secure the dressing in place to prevent leakage. (A light switchable adhesive is one the adhesion of which is reduced by photocuring. Its use can be beneficial in reducing the trauma of removal of the dressing.) Thus, the backing layer may have a flange or lip extending around the proximal face of the backing layer, of a transparent or translucent material (for which it will be understood that materials that are listed above are amongst those that are suitable). This bears a film of a light switchable adhesive to secure the dressing in place to prevent leakage on its proximal face, and a layer of opaque material on its distal face. To remove the dressing and not cause excessive trauma in removal of the dressing, the layer of opaque material on the distal face of the flange or lip extending around the proximal wound is removed prior to application of radiation of an appropriate wavelength to the flange or lip. The periphery of the wound dressing, outside the relatively fluid-tight seal, that bears an adhesive film to attach it to the skin around the wound, may be of a material that has a high moisture vapour permeability or is a switchable material. In such case, the adhesive film, if continuous, should also have a high or switchable moisture vapour permeability, e.g. be an adhesive such as used in Smith & Nephew's Allevyn™, IV3000™ and OpSite™ dressings. Where a vacuum, is applied to hold the wound dressing in place in a fluid-tight seal around the periphery of the wound-facing face of the wound dressing, the wound dressing may be provided with a silicone flange or lip to seal the dressing around the wound. This removes the need for adhesives and associated trauma to the patient's skin. Where the interior of, and the flow of irrigant and/or wound exudate to and through, the dressing is under any significant positive pressure, which will tend to act at peripheral points to lift and remove the dressing off the skin around the wound. In such use of the apparatus, it may thus be necessary to provide means for forming and maintaining such a seal or closure over the wound against such positive pressure on the wound, to act at peripheral points for this purpose. Examples of such means include light switchable adhesives, as above, to secure the dressing in place to prevent leakage. Since the adhesion of a light switchable adhesive is reduced by photocuring, thereby reducing the trauma of removal of the dressing, a film of a more aggressive adhesive may be used, e.g. on a flange, as above. Examples of suitable fluid adhesives for use in more extreme conditions where trauma to the patient's skin is tolerable include ones that consist essentially of cyanoacrylate and like tissue adhesives, applied around the edges of the wound and/or the proximal face of the backing layer of the wound dressing, e.g. on a flange or lip. Further suitable examples of such means include adhesive (e.g. with pressure-sensitive adhesive) and non-adhesive, and elastic and non-elastic straps, bands, loops, strips, ties, bandages, e.g. compression bandages, sheets, covers, sleeves, jackets, sheathes, wraps, stockings and hose, e.g. elastic tubular hose or elastic tubular stockings that are a compressive fit over a limb wound to apply suitable pressure to it when the therapy is applied in this way; and inflatable cuffs, sleeves, jackets, trousers, sheathes, wraps, stockings and hose that are a compressive fit over a limb wound to apply suitable pressure to it when the therapy is applied in this way. Such means may each be laid out over the wound dressing to extend beyond the periphery of the backing layer of the wound dressing, and as appropriate will be adhered or otherwise secured to the skin around the wound and/or itself and as appropriate will apply compression (e.g. with elastic bandages, stockings) to a degree that is sufficient to hold the wound dressing in place in a fluid-tight seal around the periphery of the wound. Such means may each be integral with the other components of the dressing, in particular the backing layer. Alternatively, it may be permanently attached or releasably attached to the dressing, in particular the backing layer, with an adhesive film, for example, or these components may be a Velcro™, push snap or twist-lock fit with each other. The means and the dressing may be separate structures, permanently unattached to each other. In a more suitable layout for higher positive pressures on the wound, a stiff flange or lip extends around the periphery of the proximal face of the backing layer of the wound dressing as hereinbefore defined. The flange or lip is concave on its proximal face to define a peripheral channel or conduit. It has a suction outlet that passes through the flange or lip to communicate with the channel or conduit and may be connected to a device for applying a vacuum, such as a pump or a piped supply of vacuum. The backing layer may be integral with or attached, for example by heat-sealing, to the flange or lip extending around its proximal face. To form the relatively fluid-tight seal or closure over a wound that is needed and to prevent passage of irrigant and/or exudate under the periphery of the wound-facing face of the wound dressing, in use of the apparatus, the dressing is set on the skin around the wound. The device then applies a vacuum to the interior of the flange or lip, thus forming and maintaining a seal or closure acting at peripheral points around the wound against the positive pressure on the wound. With all the foregoing means of attachment, and means for forming and maintaining a seal or closure over the wound, against positive or negative pressure on the wound at peripheral points around the wound, the wound dressing sealing periphery is preferably of a generally round shape, such as an ellipse, and in particular circular. To form the relatively fluid-tight seal or closure over a wound and around the inlet pipe(s) and outlet pipe(s) at the point at which they pass through and/or under the wound-facing face, the backing layer may be integral with these other components. The components may alternatively just be a push, snap or twist-lock fit with each other, or adhered or heat-sealed together. The or each inlet pipe or outlet pipe may be in the form of an aperture, such as a funnel, hole, opening, orifice, luer, slot or port for connection as a female member respectively to a mating end of a fluid recirculation tube and/or fluid supply tube (optionally or as necessary via means for forming a tube, pipe or hose, or nozzle); or a fluid offtake tube. Where the components are integral they will usually be made of the same material (for which it will be understood that materials that are listed above are amongst those that are suitable). Where, alternatively, they are a push, snap or twist-lock fit, the may be of the same material or of different materials. In either case, materials that are listed above are amongst those that are suitable for all the components. The or each pipe will generally pass through, rather than under the backing layer. In such case, the backing layer may often have a rigid and/or resiliently inflexible or stiff area to resist any substantial play between the or each pipe and the or each mating tube, or deformation under pressure in any direction. It may often be stiffened, reinforced or otherwise strengthened by a boss projecting distally (outwardly from the wound) around each relevant tube, pipe or hose, or nozzle, hole, opening, orifice, luer, slot or port for connection to a mating end of a fluid recirculation tube and/or fluid supply tube or fluid offtake tube. Alternatively or additionally, where appropriate the backing layer may have a stiff flange or lip extending around the proximal face of the backing layer to stiffen, reinforce or otherwise strengthen the backing layer. The wound dressing may not comprise any integer under the backing layer in the wound in use, other than the ribs or ridges mentioned herein. To be suitable for use, in particular to impose pressure on the wound bed, a) the interior of the wound dressing should conform to the wound bed, even for a wound in a highly exuding state, and b) the wound facing face of the dressing, e.g. a fluid-inflatable module as described hereinbefore, should contact or lie very close to the wound bed. In particular with relatively high concentrations of materials that are deleterious to wound healing, it may be advantageous to provide a system where wound irrigant and/or wound exudate may be distributed more evenly, or pass in a more linear path under the dressing over the wound bed. Accordingly, one form of the dressing is provided with a wound filler under the backing layer. This is favourably a resiliently flexible, e.g. elastomeric, and preferably soft, structure with good conformability to wound shape. It is urged by its own resilience against the backing layer to apply gentle pressure on the wound bed. The wound filler may be integral with the other components of the dressing, in particular the backing layer. Alternatively, it may be permanently attached to them/it, with an adhesive film, for example, or by heat-sealing, e.g. to a flange or lip extending from the proximal face, so as not to disrupt the relatively fluid-tight seal or closure over the wound that is needed. Less usually, the wound filler is releasably attached to the backing layer, with an adhesive film, for example, or these components may be a push, snap or twist-lock fit with each other. The wound filler and the backing layer may be separate structures, permanently unattached to each other. If there is a fluid-inflatable module that lies in the wound in use, such as a fluid-inflatable irrigant inlet manifold comprised in the dressing as described hereinafter in greater detail, then the wound filler may be or comprise a solid integer, favourably a resiliently flexible, e.g. elastomeric, and preferably soft, structure with good conformability to wound shape. Examples of suitable forms of such wound fillers are foams formed of a suitable material, e.g. a resilient thermoplastic. Preferred materials for the present wound dressing include reticulated filtration polyurethane foams with small apertures or pores. It may also be in the form of, or comprise one or more conformable non-inflatable hollow bodies defined by a film, sheet or membrane, such as a bag, chamber, pouch or other structure, filled with a fluid or solid that urges it to the wound shape. Examples of suitable fluids for filling such non-inflatable hollow body or bodies defined by a film, sheet or membrane include gases, such as air, nitrogen and argon, more usually air, at a small positive pressure above atmospheric; and liquids, such as water, saline. Examples also include gels, such as silicone gels, e.g. CaviCare™ gel, or preferably cellulosic gels, for example hydrophilic cross-linked cellulosic gels, such as Intrasite™ cross-linked materials. Examples also include aerosol foams, where the gaseous phase of the aerosol system is air or an inert gas, such as nitrogen or argon, more usually air, at a small positive pressure above atmospheric; and solid particulates, such as plastics crumbs or beads. If there is no fluid-inflatable, module the filler should be or comprise a fluid-inflatable module. It may be in the form of, or comprise one or more conformable hollow bodies defined by a film, sheet or membrane, such as a bag, chamber, pouch or other structure, filled with a fluid or solid that urges it to the wound shape. It will be stimulated to change shape as appropriate and optionally at desired frequencies by inflation with fluid, followed as desired by deflation, as described hereinbefore. This is capable of maintaining the pressure on the wound bed and optionally tissue surrounding the wound and/or the fluid thereover at a constant level. More usually, however, it is also capable of regularly or randomly (preferably cyclically) varying and/or pulsing the pressure applied to the wound, all at or near an appropriate, desired level of stress to the wound bed and optionally tissue surrounding the wound, to an appropriate, desired programme while moving fluid over the wound bed The film, sheet or membrane of such a filler often has a (generally uniform) thickness similar to that of films or sheets used in conventional wound dressing backing layers. That is, up to 100 micron, preferably up to 50 micron, more preferably up to 25 micron, and of 10 micron minimum thickness, and is often resiliently flexible, e.g. elastomeric, and preferably soft. Such a filler is often integral with the other components of the dressing, in particular the backing layer, or permanently attached to them/it, with an adhesive film, for example, or by heat-sealing, e.g. to a flange. Examples of suitable fluids for inflating the hollow body or bodies defined by a film, sheet or membrane include gases, such as air, nitrogen and argon, more usually air, at a small positive pressure above atmospheric; and liquids, such as water, saline. Examples also include gels, such as silicone gels, e.g. CaviCare™ gel, or preferably cellulosic gels, for example hydrophilic cross-linked cellulosic gels, such as Intrasite™ cross-linked materials. Examples also include aerosol foams, where the gaseous phase of the aerosol system is air or an inert gas, such as nitrogen or argon, more usually air, at a small positive pressure above atmospheric. Of course, if the backing layer is a sufficiently conformable and/or e.g. an upwardly dished sheet, the backing layer may lie under the wound filler, rather than vice versa. In this type of layout, in order for the wound filler to urge the wound dressing towards the wound bed, it will usually have to be firmly adhered or otherwise releasably attached to the skin around the wound. This is especially the case in those embodiments where the wound filler and the backing layer are separate structures, permanently unattached to each other. In such a layout for deeper wounds when the therapy is applied in this way, the means for
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