My Glowworm Ulti.120FF,is continuously turning itself on and off?

The boiler fires up and runs ok for about a minute,before it starts to switch off and then reignite,this happening once every second.So there is a constant bumpf bumpf as the boiler reignites.The 4 neon lights on the pcb initially all light up until the problem starts,resulting in light 4 flashing on and off in unison with the relay on the pcb and the boiler igniting.I thought the problem might have been a faulty pcb relay or main solenoid fault.However,I removed the front cover and it worked pretty well normally with an occasional switch off.The vent pipe looking from the out side is clear.