Captain - As ST has mentioned I feel your problem is mainly due to moisture within the interior itself
This maybe a sunroof leak
Front windscreen leak due to crack or trim surround.
Blocked sunroof drain pipes.
Leaking or worn door rubbers
Ask them to check for these items
Sticking or and non working internal vents is certainly not unheard of but usually accompanied by strange noises behind the dash trim when you flick through the air positions.
Is the car just out of warranty by any chance ? Is it FRSH to the service schedule ?
If not pay to get it thoroughly checked by an RAC inspection or similar and have wear + tear faults recitfied to save money and hassle in the long run..
Don't forget also to check with your dealer that you car has had any recall work done..