Hope this helps:
Hi Martin, sorry you have a stain on your mattress. Here are some hints on how to remove them. Good Luck and Merry Christmas. Removing Urine Stains - How to Remove Urine Stains - Spot Removal ...
How to Remove Urine Stains from Upholstery. Blot the urine stain using a cloth. Mix one tablespoon of dishwashing liquid with two cups of cold water in a small bowl. Dip a clean white cloth into the solution. Using a dry cloth (with no solution), gently blot the affected area until it's dry.
When you get them out buy a water proof mattress pad these will protect mattress from being ruined
This article may help: https://www.cleanipedia.com/gb/laundry/how-to-clean-urine-stains-from-a-mattress.html