Worcester Greenstar combi Heating works, but doesn't (??!)?

I have a Worcester Bosch Greenstar 30Si combi boiler with a Drayton SCR and RF3 digistat controls. The heating works ok when the Drayton kit is bypassed, but when connected, it doesn't. The Drayton kit appears to be fine - receiver clicks and light comes on in response to signal from digistat, but the boiler doesn't fire up on demand - I wonder if its a problem with the PCB or the Drayton kit but am open to suggestions. Answers on a postcard please...