OK then:
Class A to C airspace - Mode S compulsory.
Class D - Mode S compulsory under IFR if > 2000kg. Mode C compulsory under VFR or under IFR if < 2000 kg.
Class E to G - Mode C compulsory under IFR. Mode C optional under VFR, Mode A not permitted.
Would that be OK? - I'm meeting the CAA next week.