I am guessing but it's either bad workmanship when it was put back togeather, or even a cheap head gasket. Basically I spun a bearing last year and bought another bottom end as my old one was cracked where the bearing went, the same as what happened to ashy!
Then I took it to an old friend who rebuilds engines (used to do them well when I was younger) anyway the only other thing I can think of is whether the head needed a skim.. either way it's all got to come apart again (after just 300miles!!) but I need to decided what to do.. i.e take it somewhere expensive and get it done with a reciept!! take it somewhere cheaper and hope they do it properly or just flog the whole thing or break it.. just not sure.. as Andy says maybe I just need to cool off