Hello Chris,
Purely by chance I've just noticed a product on the Thompson & Morgan website. It says it's safe for pets, and you'll find it listed under the category "other". Good luck.

thanks very much, I'll give it a go.....

Hi Chris and welcome to GoY. Is there a reason why you pressure wash your paving and patio? We sweep ours but would not consider putting the hose on, destroys the patina and makes the pavers look new and raw again.

I use a product called Algon, available from most garden centres. Apply with a sprayer or watering can and it gets rid of algae and the build up of slime on paving. It is organic and non toxic. I have used it in previous years and my cats are fine. It takes a couple of weeks to show results though.