Chances are you have mice living in the air conditioning somewhere.
We had the same problem. When the garage was dismantling the system a live mouse jumped out and hid in their workshop. I like to think he's still there, building things at night like the mice in Bagpuss do.
water is collecting in the system somewhere, bacteria are breeding in it which is causing the smell. Change the pollen filter and, if that doesn't work, try one of those smoke bomb thingies the air-con mechanics use.
If a cat has peed down the scuttle panel/grate then go to Pets at Home and buy Simple Urine/odore liquid £8. Works great round the house if you have cats/dogs.
