potterton have modified the pcb and if your boiler is pre week33 1998 it will have a sifan fan with capacitor this must be changed if pcb replaced or capaciton will blow pcb if replacing both you will need a harness kit (5113950)
befor you replace expansion vessal most times this is caused by the connecting pipe from vessal to boiler blocking up de pressure vessal remove and clean pipe recharge vessal & refill
