Rover 414 temperature gauge rises to top.What can be done?

My dad bought a Rover 414,it appeared in very good condition.Since he bought it a few months back,he's had a few minor faults with it.Now the latest is the engine seems to be over heating.To stop it from over heating fully,he puts the heater on high which lowers the temp.gauge a bit.He took it to a garage where they replaced the thermostat.He comes away with the car,then finds the next day the temperature gauge has risen again,about 15 after starting up,it rises straight to the top,just like before.On contacting the garage,they said,they had checked it afterwards and it was ok.In the meantime they've told him to just keep switching on the heater,and if it overheats properly then bring it back.This is just no acceptable.

Does anybody have any idea what is causing this to happen? There is no water leaks at all,or any steam coming from the radiater ,and having loosened the nut,that supposedly let pout any steam,no steam comes out and its cold.He's tried driving normally without applying the heater but he can then feel the engine heating up.Any ideas?