To answer a couple of questions - and ask one more:

The spark plugs looked perfect - no suspicion of burning oil.

On emissions test, it was running closed loop but the cat wouldn't light up. I'm pretty sure I'm going to need a cat but wouldn't fit one before the engine was running properly. I'd considered it being blocked but the engine runs fine when flat out at 5-6000 rpm. Surely a blocked cat wouldn't behave like that?

If the cam timing jumps, does it throw up a related code? The symptoms fit but with sensors on both cams I thought it might notice if something jumped a tooth.