1. A method of calibrating an individual sensor of a particular sensor type whose output varies non-linearly with at least one measured quantity and at least one operating condition, comprising the steps of: a) producing a set of calibration curves for a sample sensor of the particular sensor type, each curve showing the variation of the sample sensor output with the at least one measured quantity at a different value of the operating condition for the particular sensor type of said individual sensor, said set of curves covering the required operating range for the particular sensor type; b) repeating step (a) a plurality of times with a plurality of different sample sensors of the particular sensor type so as to produce a set of calibration curves for each sample sensor; c) averaging said the plurality of sets of calibration curves of the plurality of sensors derived from step (b) and using the result to produce a generic calibration surface for the particular sensor type showing the variation of the sensor reading with the at least one measured quantity across the required operating range; d) taking individual calibration measurements for a number of different values of the measured quantity for the individual sensor at just a small number of discrete values for the at least one operating condition which fall within the full range of operating values for the at least one operating condition for which the sensor is to be calibrated; and e) using said individual calibration readings to map the generic calibration surface to the individual calibration measurements of the individual sensor in order to fit it to the individual sensor.