Have you tried using the screen demist button with the air-con on - worked a treat on my previous 350's (and now in the 370)
Might be worth turning off the re-circulating option as well (the indicator light in off position).
See 4.9-4.11 of the owner's handbook (2006 version)
If doing that there is no improvement then a regas sounds a possibilty. Shouldn't have to use RainX anti-fog, or whathaveyou, in the ZEDs