Sounds like you have a particularly persistent bacterial build up. First port of call is to get the car up to temp, then run the aircon as hot as possible for 10-15 minutes, longer if you can. The idea is to purge all the ducts. If this doesn't work, and neither have aircon 'bombs', I think you'll be looking at a professional service & purge.
As always, the system is prone to this if not run on a weekly basis at least, but it can happen even on more regularly run systems.
Fingers crossed you purge the pong, Stealth.