potterton prima f60 igniter problem?

i have a potterton prima f60, it will not fire up on its own, whilst listening to the boiler you can hear the fan start but instead of igniting it just stops and then 5 seconds later will start up again, the only way i have been able to make the boiler fire up is to hit the boiler whilst the fan is running, even then the boiler will cut out when temp is reached and will have to go through the same routine. is this a new pcb or igniter switch or something else, help