Please help me determine the part I need to fix my Prima 50F boiler ?

My Prima 50F boiler is not running properly.
When the timer calls the boiler to switch on, the fan kicks in but the boiler does not ignite causing the fan to cycle on and off. The boiler does eventually ignite but the time it takes is getting longer and sometimes over an hour.
Has anyone else experienced similar problems?
If so, was the Pressure switch faulty? Or, was it related to the Electronic Control/ PCB ?
I have bought replacemenet capacitors from Maplins just in case.
Any assistance appreciated.