Ah,this morning I was reading my catalogue that I buy bird food and bat boxes etc from (CJ Wildlife) and noticed a product called Cat Watch deterrant, its a sonic (I think) thing that you put in the ground, it says its the best thing on the market. Its not cheap at about £59, but if it worked....
I was considering buying the FoxWatch Deterrant but unfortunately that also repels dogs, so would be rather unfair on my labrador
http://www.birdfood.co.uk/ctrl/product_group:100;/catwatch_deterrent#.U02TjFVdXz8