I couldn't track down the problem myself so I took it into the garage (convenient having one right next door to empeg!) - as I feared the head gasket has blown. Damn.

I'll be replacing it with an HKS metal gasket, otherwise the chances are it will just blow again. I also need to get a really good head skim, so if anyone can recommend a good machine shop in the UK let me know.

I'm keeping my fingers crossed that the head hasn't warped, and that there isn't any surface damage to the block. Of course with my luck...

Rob