The short answer is that I don't really know, but is it possible to remove some or all of it to be sure? It sounds awful - and it must smell nasty. Yuk! Poor you.

Cat litter can actually help plants as some of them are just baked clay and acts similar to Perlite. I would mix it in then add a watering can full of water in to dilute the pee and flush it out.

I made the mistake about 6yrs ago of emptying my cat tray on one of my gardens during the winter months, it was the expensive litter that absorbs the smell and moisture, big mistake as it worked too good and also dried up the soil, I soon regretted it and that particular bed was useless for at least 2yrs after, even the weeds wouldn't grow, I have spent the years since incorporating the contents of my compost bins back into the soil to bring the goodness and moisture back, we garden on clay so with the cat litter and my compost it helped break that up and I now have some marvellous flowers growing there, so some good did come out of my mistake but OH BOY,I 'll never do that again not even the ones that say they are garden friendly.....If yours has been down for a long time you might be okay, you'll know if the weeds grow that its going to be ok, I have my own pets so know where they are trained to go but I do feel for you with this situation...

actully if any cats have peed there then that just provides nuturiants and it almost acts like a fertliser after all natural forests take nutriants from leaves,animal waste or even dead animals! but i would still recomend removing the litter as it absorbs water!
happy to help :)

Thank you for your replies. I don't feel quite so shell shocked about it now. Good job i have good gardening gloves and wellies. I'll let you all know results as time goes by.

Yes for goodness sake be careful - you can catch toxoplasmosis from cat litter, which is especially dangerous if you are pregnant - this happened to someone I know a few years back.

Goodness Cammomile, I didn't know this. I will take care.
Thank you

Update.
I managed to dig 2 deepish holes and raked in some of the offending matter to bury it. Still have loads raked up into a mound in a corner. A friend gave me some top soil which I have spread over 1/2 the area.
I'm playing with the idea of having a go at making some raised beds and narrow gravel paths. Can anyone suggest a recycled item that would be suitable for a raised bed.

Some people use pallets.

Thanks Spritzhenry

You will need a 'pallet breaker' from any B&Q (and etc) to break up a pallet. It saves a lot of time.