To provide an update/closure to my original question for future searchers:-
Unfortunately I did not receive a definitive answer. The CAA said "If you are legal to fly it in the US on your FAA ticket, then you can fly it in the UK." This may be a starting point for anyone else with more perseverance than I.
I ended up buying a share in a small PFA type (Kitfox) which is registered as a standard "Group A" aircraft. This gives me flying with the same outlay and running costs as a microlight, but without the hassle of having to get an NPPL. I fly it on the standard FAA cert & 3rd class medical.
Al