If you find that your air-con is starting to smell when used, it is probably
due a clean!, when in use bacteria starts to grow on the moisture that is left in the pipes
as the system starts to de-humidify, it is this bacteria which produces a musty smell in the car.
Buy air-conditioning cleaner,(the Comma kit shown is available from Halfords for £11.99. Alternatively Tesco do an equivalent for £4.97)
Take out EVERYTHING you can from the car, including the stereo face if possible, and make sure you have plenty of clean cloths!
If you have a combined filter, this needs to be removed first.
Set the aircon on full.
Start the car.
Set the fan to dash vents, full blower, and recirculate.