Potterton Kingfisher Bolier RS80, Firing on/off constantly?

Hi, I have a Kingfisher rs80 boiler about 4 years old. It works fine until it gets to the cut off temperature set by the bolier temperature control. Then it clicks on and off constantly. If I increase the temperature control the clicking stops until the bolier reaches the new higher temperature and then the problems re-occurs. Same if I reduce the temperature control. The noise seems to be coming from the PCB. Am I correct in that this is most likely the pcb, temperature sensor, overheat thermostat or temperature control. Is there any way of testing these?