Thermostat stuck or system air locked. Change stat. then fill and bleed system as per handbook/manual. Hope you have not done any head damage due to overheating !
on further invesigation,it loses water,But i can't see where from,and when you take the cap off the expanshion tank after a couple of minutes running it gushes out like a fountain...could it be the pump as mentioned.thanks
I agree, it is more than likely that the head gasket has ruptured,but I would still check if the stat.or water pump had failed and caused it while the head is being done.
Expensive job if you can't do it yourself.