I have two of these fitted (c 18 years ago).  

One seems to have a problem - the one feeding the domestic hw cylinder.  The manual slide lever is loose....which normally means that the valve motor has been energised and the valve opened (which it hasn't as the ch system is switched off).

When I switched the ch on and asked for hw, although a quick mains test showed volts going up to the valve, no sound of the motor running up nor any volts coming back from the valve 'open' position (used to fire up the boiler/pump).

Is this indicative of a faulty motor or faulty valve?

Thanks for the help, Scruff.  Will look into how easy/difficult it's going to be.  Bit of inconsiderate boxing in by yours truly to consider.