I know it seems a bit extreme (but desperate times require desperate measures) but if I was in your position, I'd do what I could to keep the blighters out - block any holes in fences and at bottom of gate, etc and put them strips along the tops of your fence.
Can you not borrow someone's dog for a week or so and leave it outside during the day? When the cats find out there's a dog present, I'm sure it'll deter them from coming back.
Where there's a will there's a way - you've just gotta perservere :)