Gents,
Points all noted and appreciated. I am concentrating upon the aircraft, but there are some very experienced FIs who will be looking at my results and considering how they apply to the pilot issue.
I've had that with C150 carb heat also, but don't think it's a major player - but I'm continuing to look at all avenues. It's not a simple problem, and gets more complex every time I go further into it!
All and any further thoughts always appreciated.
G