According to my reference book you need a specialist cleaner. Recommendation is Bissell Pet Stain & Odour Removal cleaner. Seems you get it from pet shops.
Buy a bottle of Morning Fresh washing up liquid (Sainsburys) for less than a pound and dilute with water say 50/50 and scrub with a nailbrush or some such. Repeat if it doesn't work first time and increase level of MF. Which chose this as the winner for stain removal over other specialist products even though it's just washing up liquid. Give it a go, for a pound you can't lose, and you can then use the rest for washing up. Sorted.