Does the hot water demand light (the one with the little tap by it) light up? If not, you have a problem with either the diverter valve (most common on these) or the microswitch on the end of the valve.
If it DOES light up then there is more amiss than meets the eye, could be a sensor or board fault or there could be a temperature-related reason for the boiler not firing.
Bunslinger, March 2009
