Hi, two things you might try. The first is a product called scent off pellets, which I have tried and they do have some success. The second is something called Lions Roar, which I heard about of the radio. It is ... how do I put this politely ... lion dung which deters cats from coming into your garden. I have never seen or tried this product, so I don't know if it really works.
get some empty plastic bottles large ones pop bottles are best, remove the label and fill them with water and place them in your garden, this type of detterent works best at night as the water reflects and the predator thinks it is another cat as cats are terrotorial well it worked for me