Baxi Combi 80e

[Dagman]

Hi, our heating system (Baxi Combo 80e) has developed a fault. When we run the hot water tap we get quite warm but not very hot water, however the radiators heat up as well! I have checked the micro switch and it has a 6v supply across it when there is no tapwater demand, when a tap is ran, the valve stem does come out and switches the microswitch off. Everything screams to me that it must be the diverter valve but, with the stem coming out it puts doubt in my mind. I don't want to change the valve to find it's something simpler. Anyone have experience with this fault. Many thanks.