So, I pulled the head and had a look, and I'm still not really sure what's up. Some of these shots are a little blurry, sorry 'bout that my hands aren't always steady.
It looks like there might be a bit of a leak in the head gasket, on the exhaust side (first pic).
The top of the gasket (second pic)
Top of the jug and bottom of the head gasket (no idea which way around it was facing, it dropped and cartwheeled across the floor)
Looking down the cylinder (more pics from other angles later)
Top of the piston at top of stroke