Well, the engineer from help-link came out today (I forgot we have insurance cover earlier). Decided that the fault wasn't in the mechanicals but was an electrical fault as the boiler switched on OK when selecting H/W but not when selecting C/H.
We await the advent of the electrician with bated breath
I imagine it is opened to heating slightly as we get one or two warm rads when the water heating is on. Apparently, the pump runs for one second or so when the heating is switched on then stops.
When the thermostat calls for heat this give power to the motor. the motor drives across, opens a paddle in the waterway and at the same time makes contact with a micrswitch, this microswitch then gives power to the pump and boiler. Pump and boiler come on and water is sent via the port opened in the valve to the rads, little bit more to it than that but in essence that's it
The microswitches inside can fail. Slightly worse if they fail on as one of mine did. Couldn't understand why the boiler was running when we went to bed one night. Boiler hot but rads and HW off. Boiler would fire for a few seconds but with no heat being delivered anywhere, switch of again quite quickly. Sometimes the motors can be a problem too and not have enough power to hold the valve operated.
I replaced the zone valves as I was draining the system to do some other work, but you can usually get replacement heads with motor and switch, or if it's just the motor, they can be obtained for some valves.