Have a main combi HE boiler. The heating is permanently on even I switch to hot water only. The heating does not switch off with the room stat it just keeps going off and on.
most likely a faulty diverter valve. it most likely stuck in the on postion. check the installers manual on the diverter valve picture. and have a look for leaks also disconnect the micro switch for the heating and see if it makes a diffrence
