How can I encourage my cats NOT to poo in my neighbours garden?

[RightUpMyRue]

I have two cats, the neighbours have no pets but they do have gravel on their front garden and a portion of their back garden has bark chips both of which are bog heaven for my cats. There are quite few cats round here and I'm sure not every poo is only from my cats but some are bound to be aren't they?

I really really don't want a litter tray and besides, they're outside cats so I'm pretty sure they'd only keep on pooing outside as well as pooing in the tray so I'd only end up with a box of poo in the house and my neighbours would still be, understandably, pissed off.

I have given them some bright green, smelly gel stuff I've had success with in the past but they don't appear to have used it so perhaps they think it's not a good enough effort on my part?

What can I do? I like my neighbours, they're really nice people but I love my cats too and want them to have the freedom they're used to (and I don't want a box of pees and poos in my house!)

You could perhaps have a litter tray outside the back door? With some sort of cover for when it rains. Or maybe outside but under cover somehow.

Get a sandpit and forget to put the lid on. That will do the trick.

Or dig over a small area and leave it as fresh earth to tempt them to poo.

I had thought about creating a nice outside loo for them down the bottom of my own garden but there are lots of cats round here and I really don't want all of them crapping in my garden too!

An outside litterbox would have the same problem except I'd have to empty it! Eurgh!!

We've bought them one of those sonic things which repels cats. I'm not sure what more I can do....?