I've trawled the forums looking for someone who has my symptoms and I can't find one that fits.
The problem is with the central vents. When I adjust the temperature so that the car is providing temperate/cold air, they work fine. When I increase the temperature so that it's providing heat, all air from the vents stops... until the ambient temperature is reached and the air con goes back to proving temperate air, in which case it starts again.
It doesn't matter whether the engine is cold/warm or how long the air con has been on, it will do it whenever the temp is increased.
I'm guessing it's a flap, but I didn't realise that there were 2 flaps - one for hot, one for cold?
Can anyone shed any light on this for me please?