Housewarmer 55 locking out?

Hi, I have a potterton housewarmer 55 back boiler that keeps going to lock out. It seems as though it only happens once the hot water has gotten to the temperature its set to. If i empty the hot water tank, then it will reignite when i press the reset button. I have looked through the service manual and it seems that after reading that, common sense would tell you its either the Control thermostat, or the overheat thermostat (im guessing the former). Any input would be good before i ring a gas man so i can at least pretend to know what im on about. Thanks