Hi, Mr Gas man is giving false advice sorry, the reason why your boiler is locking out and says "overheat thermostat" is because the boiler has over heated and has shut down which is a safety device. Your heatline boiler has two thermostats, (1) limit safety thermostat (2) overheat thermostat. From what you were describing the limit safety thermostat has failed and the No 2 overheat thermostat has kicked in.
These can be easily replaced and should cost no more than ÂŁ20 depending on were you shop.
The limit safety thermostat is found near the bottom left hand side of boiler.