It's either your gas valve or the PCB.
There are two grey wires at the base of the gas valve,check you are getting 24 volts at these.If you are'nt it's your PCB.If you are your gas valve is probably stuck.You could try giving it a tap as the boiler is going through it's start up cycle,failing that a new gas valve is the order of the day.
Josh, November 2008
