HW working ok. CH not working! Last week CH started to be temperamental. Heating switched on and rads hot then all of a sudden heating still on on integral clock but rads would go cold. Then would switch heating off on clock but pump would keep running for hours. Now heating will only fire up for about 30 seconds then overheats. Pump seems to be ok and checked diverter pin seemed to be moving freely, Checked system for air. I am a gas fitter but not very good at breakdowns, any ideas what to check next? ha. Any help would be much appreciated.