I have a weird problem. I have Manual Heater if I select face vents for example, number 5 on the fan and the car is ticking over a howling gale comes out of the vent  , Just what I was expecting but as soon as I press the accelerator the gale reduces to a light breeze and the screen demister starts working. Lift off the throttle ever so slightly or let her get back to tick over and the flap closes the demister and back to normal gale with no air through the screen demister. The same happens on all the other settings apart from the obvious Screen demist

I only got the car in November and for most of the time it has been set to demist or the fans were running light so I didn't notice the drop in force. Checked it out again on the run to work and with Aircon running exactly the same symptoms. On my previous saloon you could hear the hiss and the flaps whomp into place as you turned the dial this one don't have that sound

Any ideas's  

Quote from the manual:

"If the electronic air conditioning system is
not functioning in the usual manner: press
the AUTO and OFF buttons simultaneously
for approx. 5 seconds. The servomotors will
be synchronized, during which time the fan
will temporarily switch off."

Might do the job.

Classic symptom of a vacuum fault.

In the plennum by pollen filter is a vacuum reservoir, it has two connections, the one which is recessed has an in-built non-return valve and connects to the brake servo vacuum pipe, this is the 'input'.
The second connection is the 'output' and supplies the heater panel via a connection inbetween the two heater pipes on the bulkhead.

The symptom you have would indicate a faulty non-return valve or a leak on the output side of the reservoir. Favourite place for a leak is where the vac pipes pass through wall into pollen filter chamber.....they can wear away here, check the full length of the vac pipes for any damage or worn areas due to rubbing.

Can also leak on the cabin side but much harder to find.....may be able to hear a hiss