Thanks guys - problem solved!
Turned out the metal arm on the valve itself wasn't attached to the plastic actuator. So although the knob turned OK and the cable worked the actuator, it didn't control the valve. A little judicious adjustment in the style of the Jeremy Clarkson school of precision engineering and Viola! Hot and COLD running air.
Now all I have to do is fix the brakes, the roof, the rubber seals, the door leaks, the...........
ps. Viola is a French word for a type of plastic flooring.....I think?