The best stuff that I have found is called Odour Eliminator. It is usually sold by vets for getting rid of the smells from pet accidents. It uses enzymes to get rid of the organic material. Spray it on liberally, then keep the area damp for a day or so. If all else fails, this one will work.
I think it costs a fiver or so.
Some pet shops sell similar stuff, but the vet supplied one is the best.
You really don't want to know how I decided that this stuff is so good.
And no, I'm not that ancient!