I had seen that (very scary) thread but I'd discounted the possibility of something as serious as a burnt valve because the EML has stayed off and nothing has shown up in the ECU log. Also the car idles smoothly...
I think it would be good to eliminate something that serious though so I'll get hold of a guage and report back.
Someone has also suggested that it may be a small vacuum leak somewhere. Do you have any tips on the best way to track one of those down? With the engine bay being so tight, it's quite hard to get to all of the induction system.
I'm not a mechanic but I'm willing to learn!