^ what he said!
about a minute from ending your journey, crank the temp up to high and switch function to auto for a bit, then to demist for a bit and finally recirculate for a bit.
what this does is dries out the ventilation ducts so no damp stale air is trapped causing a bacteria build up.
i had this problem last year, tried using a bomb but did not work!
i tried this method that i discovered on here and never had the problem since!
i also read that using the econ mode button is bad, as sometimes this can cause problems with the aircon not switching back on again and forever stuck in econ mode as the compressor clutch will not engage! very expensive to buy a new aircon compressor, and a pita to change the clutch assembly.
so to summarise, don't use the econ button, and heat your pipes before switching off and you will have sweet smelling aircon!
also, i got mine regassed last winter at kwikfit on offer for £29 as nobody uses cold aircon in the winter! they do a policy that if it will not charge up because of a fault or leak, there is no cost to you.
mine charged up ok and over the last few months has been a god send! ice cold and lovely!
it is recommended to have the aircon gassed annually btw.