I'm sure this has been answered before here, but some suggestions are....get your own cat, get a dog, get some lion poo! Also you can grow some cat-unfriendly plants such as lemon balm as they hate the smell.
Go to B&Q and buy some green cat pellets, cats don't go near your garden once they're down, I should know as I asked the question the last time and this was suggested, I've had no problems since other than it doesn't look to good having all these green pellets in your soil. I've also just bought cat dust which is the same idea only not as noticeable although I've not tried it yet.
Where's Spellmaster when you need him? ollie - when a cat "fowls" in your garden, is it doing an impression of a chicken? Or is it fouling your garden?!
Try used Tea bags with a few drops of Tea Tree oil on them - they last for a good few days and are environmentally friendly. Other option is to use pebbles on the border - not a truly environmentally friendly option I know - but one i had to resort to before i found out about the tea bag thing...
My missus uses plastic cutlery forks & knives it looks ridiculous but is very effective they dont rot away and the cats do their business somewhere else
Moth balls are incredibly effective albeit they look a bit odd on your lawn, or plastic bottles filled with water. They reflect the light and they won't go near them.