My boiler suffered a feeble pilot. A solenoid opens the gas valve to the pilot - that might be playing up. The gas comes through an injector and that may be dirty and partially blocked (mine was). The pilot is only permitted to light when the pressure switch senses the fan running so either might be the cause. It's all controlled by a circuit board which might also be faulty. The cause is likely to be only ONE of these things.
Boiler manuals often have fault-finding charts in them.
Do not permit the heating engineer to change parts on "spec" else it may cost you a fortune.
They will all be taking an extended break soon so call a heating engineer as soon as you can.
Peccavi
December 2008