POTTERTON PRIMA F ERRATIC FAILURE TO IGNITE?

When the hot water or heating is switched on the boiler attempts to start up, i.e. the fan starts and stays on for about 15 seconds and then stops for about 10 seconds. This cycle repeats indefinately without the ignition spark appearing. we have replaced the 3 capacitors on the PCB recommended in similar advisory replies but this has made matters worse! Previously the boiler would fire up for maybe 5 minutes before going into the "fan cycling-only" routine.

Are there any trouble-shooting suggestions to help us get the ignition spark going again?
Can anyone please help!!