Yeah I did all that and scolded my hands in the process. I removed the engine yesterday and began a strip and rebuild. The problem became very clear as soon as I started to remove the head bolts - they weren't very tight!!
the top inner two (inside the inlet ports) were more or less finger tight and undone using nothing more than a half inch ratchet with little pressure. I've undone tighter starter motor bolts before. Once removed it was all very clear why the water system was pressurising.
Here's a picture of the head gasket which the last owner had fitted shortly before selling it on to me. The gasket is largely ok with no signs of major failure but you can see at the top where the spent gasses have worked there way past the sealing ring and into the water jacket
And here's one of the combustion chambers (the other one was the same) and again the dark burnt area at the top where the gasses have been forced though. How this engine ran is beyond me as it actually ran reasonably good.
Not sure how the head nuts were loose - maybe they worked loose or could it be that the last owner just simply forgot to torque them up properly? I got a new copper one on its way and I'll be fitting a whole host of new parts that I've collected aswell
