could it be a air lock

i hope it is air,head gasket dont look to hard to change tho

up date,i think ive sorted it.the pipes from expansion tank to top of rad and thermostat houseing where blocked solid

If the problem does come back(I hope not). Let's the engine run for 20 secs from cold, stop the and check for any pressure build up in the expansion tank. If not try again at 1500 - 2000 revs for 10 to 15 seconds. If there is pressure after any of this attempts chances are that its head or head gasket. The coolant shouldn't expand this quick.