I have a similar problem to Tara and having used 'Silent Roar' recently I can testify that it's really effective. You have to make sure you remove all the cat poo before being really liberal with the Silent Roar pellets. It stopped the perpetrator for a few days and it hasn't fouled again in the place where I used the pellets. Only thing is the cat is now finding other areas of the garden to mark his territory in. I'll have to go round with the Silent Roar again! Luckily, I bought two boxes of the stuff...

It's a shame that this particular cat's owner isn't a responsible one. He/she should provide the cat with a litter tray, and get the cat neutered so it's less inclined to wander and doesn't feel the need to mark its territory all the time.