Thanks for the replies, Just come back from there. As Wilpickard says, cast iron head, took it off and it seemed fine, put a straight edge all round it and looked closely for any signs of burnt valves and cracks and it looked spot on. Bores nice and clean with no signs of scoring so put it all back together with a new HG and thermostat and water pump. Annoying part was putting the pulley on the waterpump with the 3 small bolts finger tight, then fitting the serpentine belt round all the ancilliaries ( the idea was to tighten the pulley bolts after the belt was on but it was such a ******* to do that I kinda forgot)
So all on, oiled and watered and fired it up... obviously something wrong as ex was blowing and then the belt came off. My suspicion is that the bolts came off and the pulley with it. Was well fed up with it by then so have left my son to finish the job off!