Vaillant boiler won't stay 'on'?

My Vaillant CH boiler 242 EB Combi - LPG fuel from 2x87kg cylinders
Refuses to stay on; when the heating is on and I run the HW tap it clicks off immediately (red light next to ignition button comes on). It's driving me mad as the boiler is in the loft in our small cottage and I have to keep traipsing up there to switch it on again. The heat exchanger and gas outlets in the burners have been cleaned recently and all pressures are fine. Any suggestions, PLEASE!!!