The first thing to check is the pollen filter, it might not have been changed in the service.
A/C systems do get a bit smelly due to water left in the evaporator and a build up of bacteria.
To get rid of it you need to get the system sanitised/deodorised, there are many different methods of doing it, you can buy an aerosol which you put inside the car with all the windows closed, and the a/c on recirculation. Basically the vapour it produces gets taken back into the system and circulated around the car.
Have a look here
http://www.speeding.co.uk/acatalog/Air_Con_Ser vice_Products.html