OK, another update, and this will only make sense to a few people.
The system is wired in a Y plan.
The magic orange wire that puts live to the motor and boiler goes live when the tank thermostat runs the system.
It does not when the room thermostat clicks on - but there is a live getting from the room stat to the wiring in the airing cupboard.
So my conclusion is the the mid position valve is fubared and I'll have to pay a man to change it!
Thanks for everyone's help.
G