Your boiler is not a "combi" neither is it a modern style condensing boiler. Combination boilers do not have hot water tanks with immersion heaters in them which is one of the major drawbacks of combis...lucky there...
If the Fan runs and the pilot lights the fault is likely to be...
Pilot flame too feeble to send sufficient signal to control board to open main gas valve
Control PCB sensing pilot flame OK but not acting on it by opening main gas valve
Control PCB trying to open main gas valve but gas valve defective.
When the engineer comes ask him nicely to diagnose the fault and repar it and not to guess what might be wrong - some of them do this at the householder's expense. The first place he should look is at the control voltage that opens the main gas valve - if that is present it's likely to cost around £250 (guessing) for fitting a new valve. If the engineer is from British Gas he will declare the boiler obsolete and insist on fitting a new one and a power flush of the radiators £2500 or so.
Good luck...
Peccavi
March 2009
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