1. A calibration system for determining calibration data for a host device having an X-Y digitiser, the system comprising: an alignment head comprising: i) an array of magnetic field generators in which the relative position of each magnetic field generator is known; and ii) a selector operable to select a magnetic field generator of the array; a calibration controller operable to control the selector to control the selection of said magnetic field generator; a drive circuit operable to energise the selected magnetic field generator to cause the selected magnetic field generator to generate a magnetic field for interaction with the X-Y digitiser; and a calibration data generator operable: i) to receive digitiser signal values from said X-Y digitiser in response to the interaction with the magnetic field generated by the selected magnetic field generator; and ii) to generate calibration data for the X-Y digitiser using the digitiser signal values for the selected magnetic field generator and the known position of the selected magnetic field generator.