Hot Water but no Central Heating?

Hi - I have a Glow-worm Hideway. I have a control box/timer which controls both HW and CH. I usually have the HW on timer and the CH permanently set to on -controlled by a room thermostat.

Following a power cut on friday, I currently have HW but no CH.

If I switch on hot water, the boiler fires up there seems to be no problem. The room thermostat 'clicks in' when set above room temperature but the boiler does not fire up. I have reset the controller and checked batteries in the room thermostat. The pilot light does not go out. I have water in the header tank.

Please can anyone offer a suggestion?

many thanks

Rod