Do not get a cat of your own, they will kick the gibbleolliepops out of it, probably kill it,
the best solution I have come across is sprinklers on motion sensors, if placed correctly are a great deterrent, you might have to invest in a reasonably sophisticated system, one you can override at a flick of a switch, so you can use the garden, you can run these off water butt.
but well worth it if you regain your space.
I also recommend you grow something prickly on your perimeters,
www.davidaustinroses.com/english/showrose.asp?showr=658
find someone with a holly tree, offer to prune it, use the leaves to mulch all beds, it puts them off using area as a loo,
bulk buy some citronella essential oil , keep all tea bags put them on beds add citronella, add it to the water butt that supplies sprinklers.
Do invite a friends with a dog, for a walk that ends up in your garden for lunch, curious cats will a shock when encountering unexpected dog.