Same answer as I gave for another question 'Faulty ravenheat CSI?'
"I haven't checked this model boiler, but it is basically the same operation for all these types of boiler, if it reaches the point of sparking at the burner, it has done all pre-ignition checks and is now looking for gas.
I'm not trying to be patronising (you'd be amazed at how often this happens) MAKE SURE the gas is ON.
Otherwise, you'll be looking down the route of a failure (in some way) of the gas valve (solenoids etc)"
Maybe in your case, blocked pilot
Carefree, July 2010