Before you go changing valve heads Sooty, have you checked that the room stat is actually switching?
If it is, looking at the S-Plan wiring diagram there's an easy way to check each circuit.
For the radiators, switch off HW at the timer then take the cover off the 'heating' valve and turn the room stat to maximum (it helps to have an assistant). You should see the valve motor run to open the valve. If the motor doesn't run either the motor is duff/jammed, it's not getting power from the room stat or it's neutral is open circuit.
If the motor runs but the pump and boiler don't fire up, it's the microswitch or wiring to the boiler.
If you're comfortable with electrics a neon screwdriver is all you need to check for power. The signal from the valve microswitch to the boiler should be the Orange wire.
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