The answer to your question is - nothing. If a cat wants to cross your garden, it will, even if it has to climb into a tree and then drop down into it. The RSPCA has rules about how far you can go to keep animals away, so for instance, putting broken glass or mousetraps where you know a cat passes is illegal and you can be prosecuted if a cat comes to harm, or any other animal for that matter. The most effective solution I've come across is water - I have a friend (electronic engineer) who has fixed up his sprinkler to a movement detector - anything moves in the garden, the sprinkler comes on. This has been very effective.

My dog keeps cats away. There used to be quite a few about but I don't see any now during the day. There might be some at night but I think they are discouraged because I don't see any signs of toiletting.