Have a couple of rooms with light coloured carpets, cream, ok so not a good idea! Some months ago one of ours marked, no problem I thought, done this before. Nothing would remove the yellow stain, in the end in desperation threw so much at it I think we had some sort of chemical melt down and had to change the carpet.
So having given 'good' advice about this before I am now repentant and need suggestions, as he has done it again!  So far have used 'vanish' but it's still there - help?

Try a solution of biological washing liquid on the patch.

Lucy's suggestion of biological washing solution is perfect.  I use a solution of 100mls wash liquid to 1ltr of hot water and foam up as much as possible..use the water to scrub/wash as normal, blot as much liquid as possible and then rub in the foam.  place a clean white towl over this and stand on it :-) to soak up remaining liquid and then slosh some simple solution over the whole area, going outside the water soaked area.  rub in, blot dry and leave the rest to dry naturally.  so far, I have no chemical marks or water marks and no smell.  This carpet has been down just over 2 years 15 dogs and 4 litters..

Many thanks to you both, before I begin do you mean biological washing liquid or powder and Kay when you say 'some simple solution' could you explain?
He has not done this for years and can only think it's due to people staying in the house but we have people coming and going all the time? Poor boy, I called him to the room and he wouldn't come in at first, then came over to where I was standing had a sniff, before looking up at me to say 'whoops'. Firmly said 'No',  and he went back to his basket, pointless I know.

I use liquid Lacy, foams up very quickly.  Powder is just as good, but I find it takes longer to dissolve and because it is made for automatic washing machines, it does not foam up as good as the liquid.  personal preference only.  

Simple solution.  sold in PAH in 100ml, 500ml and 1lts bottles... already made up so no mixing. 

Many thanks will take a trip to PAH this afternoon, finger crossed.

Kay. It's worked, thank you. A slight pale mark, but think this just means the rest needs cleaning! Never had a problem before, whether it's the carpets or him I don't know, but then he's not wee'd inside for years.