JonathanJ, thanks for the suggestions, I think I've a compression tester somewhere, I'll just have to try and work out how to use it

Marty, the cat is something else I've considered, but like with Jonathans suggestion why would it only do it at a) low rpm and b) when it's hot?

I'd have thought something like badly seating valves or a blocked cat would be WORSE at high rpm, but the engine is sweet as to the red line.