Forgot to say that the part will cost just over £100 and then you need to get a corgi plumber to fit.
Is there low pressure in the system - is the reading on the round display at the front of the boiler below 1? If so this may also cause the same problem - add water from the valve underneath the boiler to raise the pressure, turn off the boiler, which resets it and then turn it back on - if it still doesn't give you any water then its probably the divertor valve.
Steve
steve, December 2007