Worcester MT10RF thermostat

I'll admit I'm not familiar with this device, though the Worcester RF /programmable/ thermostats I've seen are Draytons (which are a PITA, IMHO). I'd suspect that possibly the stat and the receiver at the boiler have lost communication and maybe the receiver has a fail-safe mode where it fires the boiler a certain amount of the time. (I don't know if your device can do that, but Honeywells can be programmed to do that.)

It hasn't lost communication. The boiler goes on and off if I turn the thermostat up and down. It just switches at the wrong temperature and now I know why. I thought I'd better change the battery before the British Gas engineer comes because the manual says battery life is only 1 year. It has two themostats on the front for "Day" and "Evening" linked to variable resistors (potentiometers) on the PCB. The link shafts are not correctly engaging with the pots. They slip as the thermostats are turned.