So after all that, I'm now pretty certain the head gasket has gone! But at least I have my answer at last, time to start stripping it down.
I can't wait to get it back running nice, I do love these cars, they're great fun and so comfortable to drive
Are you going for a Payne Elastomer? I believe it may not be the best if your engine is in perfect nick but is more forgiving of a less than perfect engine.