My guess is you dont have a cat or dog of your own and they are marking their territory Jemma.

This month's BBC Gardeners' World magazine (August) has a whole list of cat repellent plants.
However, I would say that my own cat waters everything in the garden and it doesn't seem to do any damage so you might have a different problem.

Sometimes cats spray can smell very strong and unpleasant, if sprayed near the house. We had a stray cat who used to spray our front porch for a few weeks and it was awful.

I really don't know if it works with cats but dried chilli flakes deter rabbits from tunneling in my pots and tubs, I'm hoping that they begin to associate the smell with 'hot feet and singed whiskers'....
A packet in the ethnic aisle of the supermarket is only £1 so sprinkle it on and around plants that need protecting.......And please let me know if it works.......

Thanks I will try the chilli seeds , it's like fight night out here, all fighting an spraying all night long you hear them, the spraying is awful. I own two dogs an one is a rotty so I have no clue why she isn't a deterrent lol hope this works but when one neighbour owns 8 cats I think it a losing battle.

You might be interested to read the postings from Gattina about using wintergreen aka Methyl salicylate which apparently deters porcupines amongst other mammals! One strategy which my OH delights in sharing, but which I have never needed to try, is to lure said cat(s) with a treat and cover the parts it likes to lick with lazy chilli. Use your imagination as to why this deters them from returning!!