It can be explained by water boiling off.... I've compression checked it today and all cylinders give 165 psi so no problem with valve sticking open etc etc. All plugs were a bit sooty (which i suppose they would be as running in) but no.3 plug was more sooty than the others. When the engine popped it was no.3 that melted. I've borrowed my mates head gasket tester kit (the chemical one where the liquid changes colour) and I'll use that tomorrow. I'm kind of expecting it to fail that test but god knows how a head gasket could go just cos its not been used.