Cheers, it did turn out to be the head gasket. But there is 1 thing still bothering me. The bottom end was rebuilt using a high pressure oil pump. When it was first run up, the oil pressure went to over 5 bar. Then the head came off again and now reads the same as before the engine let go, both hot and cold, and idling and driving. Surely a high pressure oil pump should read more than a standard one!