Hi all
Just come across your forum and hope you can help with my problem (well rather my daughters).
Had a lovely Christmas dinner then daughter realised boiler was not switching off, even though thermostat was down to below 15. We left her at 8pm and she called in panic at 11pm to say it was still running even though she has turned heating off completely..She lives in rented house and agents answer machine says "no one available until 4th Jan" .. I would be really grateful for any suggestion as to what she should try, she has 2 small children and a limited budget so is scared of calling out someone herself. 
My first guess is that you have a motorised valve that has stuck.
They should open when the thermostat calls for heat and spring closed when not.
A microswitch in the valve activates the boiler when the valve opens. If the microswitch fails or the valve sticks open, the boiler won't shut off.
There should be a spur box near the boiler to isolate the supply to the heating controls. Switching that off will stop the boiler cycling constantly.
Kevin