My neighbour has about 10 cats and they use my garden and another neighbours as their toilet - she says they don't , but we have seen them frequently. If I send it back it is an offence?
Cats poo to mark their territory and often seem to have pooing tournaments if territory is in dispute. Your neighbour cannot know where her cats do it without following each one.
Cats are very susceptible to simple aversion training. Given the size of your garden (does it have a moat? ;) ) try using a super-soaker type water pistol and play target practice with them. With an individual cat, chucking a jug of cold water over it while shouting loudly works a treat but I'm having hysterics thinking of you trying that with all 10 of em.
I think, whether it's an offence or not, I'd return it to her. Perhaps you could go around collecting it over a week in a polythene bag and then knock on her door and politely tell her her cats left their belongings behind, so you're kindly returning them.
I'm not sure what you can do longterm, no one has control of where cats will go. Perhaps you can get a dog?