Its definately your air con, alot of people say they get an eggy or farm yard smell in the cabin when either the heater (on cold) or air con is on.
[u][b][size="4"]What causes this?[/size][/b][/u]
well when you use air con, a few people will put it on recycle as the air con is less harsh, for example when i have the air con on (with recycle off) it gives me headaches, this is because its so cold, air con removes the moisture from the air by compressing it, hence the coldness.
now when your aircon has been run on recycle, even once, bacteria from when you breath is put back into the system, this then grows and bacteria when concentrated will start to smell, hence the smell of eggs or farmyard.
air con cleaner is a fix, but a temporary one.
there are 2 things to do to ensure this doesn't come back.
have your air con drained and re filled and have your filter replaced (as the filter most certainly will contain bacteria spores.
£40-60 is what you'd be looking to pay for a re-gas and another £10 for a filter if fitted yourself. if sent into a garage or fords it will cost you more say £20-25, but at fords the cost could be much higher, its not a specialist thing so any garage can do it for you.
hope this helps