Cylinder Thermostat not switching off?

I have recently had my system drained to fit a new bathroom. Since then the hot water has been too hot. I tried turning down the cylinder thermostat to 50 degrees. It had no effect, the boiler continued to heat the water. I guessed the problem was the thermostat. I have replaced it with a brand new one. It didnt solve the problem. If I gently press the bi-metal strip on the thermostat it works correctly and switched off the system. So the cylinder thermostat appears to be ok but it isnt getting hot enough to operate the switch. I have tried touching the cylinder where the thermostat usually sits and it is getting quite hot but not too hot to touch. Could it be an air lock caused when the system was refilled or is there another explanation. i am now really confused, any help would be apreciated