18. The method as claimed in claim 17 , wherein a preference value is determined by allocating a weighting to the or each such comparison, and combining the allocated weighting(s) in a predetermined manner.
18. The method as claimed in claim 17 , wherein a preference value is determined by allocating a weighting to the or each such comparison, and combining the allocated weighting(s) in a predetermined manner. 22. The method as claimed in claim 18 , wherein the allocated weightings are combined by summation.