How can i fixMy Glowworm Micron 70FF?

My 70ff Boiler has had difficulty igniting. I would find that the fan was running but the boiler/rads were cold, after some coaxing with the rest knob it would eventually ignite. I think a faulty PCB and Thermistor may be the cause? I planned to tackle the job when the weather warmed up, however the problem seemed to have stopped now that the boiler is only heating the hot water (Y Plan system). But another problem has arisen, the pump over run seems to last far longer than it use to and hot water is being pushed up the vent pipe back into the feed and expansion tank. Has anyone else had this problem and does my initial view that the PCB /thermistor are faulty still hold?

Joe