A couple of ideas...
1) Check for places the valeters might have missed e.g. the roof lining, the parcel shelf, the boot carpet, under the seats. One of them might be the source of the smell.
2) Pull back the carpet around the footwells - There might be a water leak and the underside has gone rotten.
3) Try a load of good'ol fabreeze before you try something more expensive. Autoglym do an car odour eliminator spray too.
4) Have you got a mate with a garage that you can leave it for a few days with all the doors open to give it a good airing. Might help??
5) If its really bad, it would only take a weekend to completely strip the interior and clean everything properly (but days to dry out!)