Hi Baz, Ive read with interest all the replies and I'm not getting involved with the debate on cats but out of interest having tried all the normal deterents, there is one which works but you do need to live near a zoo and its to buy a bag (locally here used to be £5 a bag) of LION poo. Being very territorial the cats wont invade the 'lions territory' for obvious reasons and the effects are quite long lasting, especially if kept covered from the rain which eventually dilutes the 'scent'.
hope this helps, best wishes Lannerman