My central heating system refuses to heat the domestic hot water although the central heating is fine. The Drayton mid position valve tell-tale at the top remains on the left and does not move over when I select "hot water only" at the control box. I have taken off the motor assembly and can turn the spindle manually through the ten degrees required and in this way I can get the hot water from the boiler to flow into the hot water cylinder. I don't know if it is significant but even when the controls are set to "Off" for heating and hot water the motor in the valve remains red hot and only cools down when I turn the whole system off at the mains. Any ideas please?