Hey Chaps

What should the thermostat control switch on and off to control the heating? Is it the valve or the pump.
It's a MegaFlo system if that helps.
I've replaced the programmer and the motorised valve and I'm back to square one... The heating comes on with the time but wont go off acording to the room stat

If I take the stat off the wall I have 2 wires, one red and one yellow. It looks like someone has cut off a blue wire and there is an earth that looks like it has also been cut

I assume the red is the live and the yellow is a switch wire? So the red goes to terminal 1 in the stat and the yellow to terminal 2. Does that sound right?

Thanks