potterton performa 24?

Having hot water issues! Heating works fine, but when hot water circuit operates the heat is applied to the heating circuit and the water runs only just off cold. If I isolate the heating flow outlet, the hot water works perfectly. I have checked the microswitches are being actuated and operating and both the pressure differential and diverter valve swiches operate as they should. Is this the diverter valve at fault and should the valve be replaced or is this usually repairable with the available repair kit?
Many thanks in advance