hi Jon, i think the smell is caused from bacteria inthe air con system.
you can buy a can of stuff (halfords?) which is like an aerosol that you release into the car when you have the air con on. that circulates the contents of the can into the air con system and cleans it and gets rid of the bad smells.
if u go to a dealer they will just do the same, but charge you for the privalege!!
if u have ur car still under warrenty then just point it out to the dealer and see if they will cure this for you. i used to point this out every year i got my car serviced at audi and they sorted it out (but then im sure it was included in the price!!!)