Think I've cracked it.
I did resort to using my 'wrist rocket' catapult filled with gravel but all it did was damage my fence and alert the neighbour I was trying to kill his cat.
I've smeared grease on the fence where the little bast.... Darlings were getting in.
Either they don't like the look of it or they tried but got in such a mess they gave up.
Either way, no more cats sh*tting in my garden.