For cats, the most effective is a sprinkler linked to a movement detector, so it switches on as soon as there's movement in the garden - not sure it'll work for foxes, don't know how they feel about getting wet, but it might work for them too. See link here for a battery version

https://www.amazon.co.uk/PestBye-Battery-Operated-Activated-Repellent/dp/B004YCUT4W

Hi
I purchased three pestbye scarers it took about three weeks for the cats realised it was them that was activating the scarers
At this moment only one visits but does not stay long enough to do its buisiness All in all I would say it works but takes a little time
Good luck
The scarers I am talking about are the sonic noise ones not the water

The ones which spray water are more effective much more quickly...

I bought the 'sonic noise' ones in the summer which have no effects on the cats & foxes. The little girl in the next garden said to me soon after I put them in , 'what's that noise mummy & I can hear in your garden' (I cannot hear it though & neither the cats) I told her what it was, and it was to scare the foxes away, didn't have the heart to say it was to scare your two cats away. I have now placed chicken wire on the bare patches of earth, its unsightly but it is working, also, its a pain when you need to do the weeding, still, not a lot of that going on now anyway. Good luck, and when you find the solution, please let us all know, lol thanks xx

I have tried a light solution of Jeyes Fluid, Obas Oil on small pieces of cloth or tissue or a sprinkling of crushed moth balls (Naphaline) is worth a try. My neighbour has been trialing Wilko Cat Repeller Granules, £1.65 for 500 grammes; it is garlic based and she reckons that it does the trick but needs to be sprinkled on a regular basis.

I sprinkled white pepper on a gravel path that was being cat fouled and haven't had a problem since.

Loupylou - that's very interesting! I know that people under about 20 years old can hear high pitched sounds which older people can't; in some towns, they trialled a high pitched whine audible to teenagers (but not the rest of us) hanging out in high streets and shopping areas in the evenings to keep them away, but a sonic cat scarer shouldn't be detectable by human ear whatever age you are - if you think about a 'silent' dog whistle, a dog can hear it, but humans can't. Seems like your sonic scarer would be good at keeping crowds of young people away instead of animals;-))

I have 3 cats and they need to go somewhere so I put a heap of compost in a corner of the garden that is out of the way and they use this. I can easily remove their jobs and replace the compost when needed.