1. A method of accessing sets of information stored in an information system, wherein portions of said sets of information are enclosed by tags of a hierarchical tag structure defined according to a structured mark-up language, the method comprising the steps of: (i) generating a search query comprising specified search criteria; (ii) identifying portions of said sets of information matching said specified search criteria, and outputting a list of references to said identified sets of information; (iii) identifying, for each matching portion identified at step (ii), an enclosing tag structure and outputting a list of said identified tag structures; (iv) receiving a selection signal specifying a tag structure from the list output at step (iii); (v) adjusting said list of references from step (ii) to comprise references only to said identified sets of information that contain the tag structure selected at step (iv); (vi) adjusting said list of tag structures to comprise tag structures contained in information sets referenced in said adjusted list at step (v); and (vii) repeating step (iv) in respect of said adjusted list of tag structures, and step (v) to identify a more specific list of references to sets of information.