how can i fix my vaillant turbo max 282?

i have a vailant vuw282 turbo max the heating system works fine, but the running water isnt getting up to temperature there is a slight leak from the diveter valve by the micro switch and the divertor operates the microswitch on and off causing the boiler to cut in and out during hot water demand. but when i tested the system by physically keeping the switch closed so the boiler wouldnt cut 'in and out' the water temperature was still not getting up to temperature. any ideas ? my only thought was the heat exchanger can this be cleaned easily.