Thank you everyone for your replies I do appreciate it! Sorry for my delayed reply! Ok, firstly, I confess I haven't thought of the engine mounts. Yes, there is a tie-bar at the top of the engine like later 3.0 GTVs imported here.
I have tried engaging/disengaging the air-con and switching off all the climate control, still no change. ( I also isolated the air-con by removing the belt to exclude that and the steering pump). I admit I haven't checked the cylinder liners too. Hope this isn't the case! Car is otherwise fantastic, no other issues at all, just the usual GTV mods to suspension and brakes that have all gone really well.
I will have a screwdriver at the ready the next time it is out and lever the support a little at idle, meantime do keep the suggestions coming.....Be nice to speak to other owners of these cars but there isn't a register that I am aware of.