Boiler not starting up?

My Potterton 80e did not automatically start up yesterday and after checking supply, fan etc. I was advised to change the controller. I did this but to no affect. However, if I turn the thermostat down to almost zero, the fan starts up and the boiler fires. The same thing happened this morning, I had to turn the thermostat right down to get the system to work. After a while I can turn the thermostat up but it is difficult to determine whether it is actually adjusting the water temperature. i.e. pretty hot all of the time. Can anybody confirm for me that this is a thermostat problem before I spend any more money.
Thanks