Yeah, i'm suspect to a crack in the turbo myself. The guy from dynamic turbos reckons it's unlikely but I think i'll get the turbo sussed out anyway. Has anyone had this problem before?
The head gasket is under a large amount of pressure normally and it's holding that fine. I find it strange that the increased pressure (with my pissy amount of boost) would be enough to suddenly leak compression into the cooling system.
But if the turbo is ok the gasket/head are the only other possible sources of the problem as far as i can see. So i'll probably get the turbo checked out (as i don't really know it's history) and then check the gasket/head after that.