What kind of relationship do you have with your neighbor? Can you ask them to keep their cat inside more, or at least to provide him with a litter box so Mr. Cat maybe won't feel such a need to use your yard in this fashion? Is the cat neutered? Neutered male cats tend to stick closer to home and not to be as into territorial marking, though once a potty habit is established, it's harder to break.
There are sprays and crystals you can buy that are repellents for cats and dogs. I live in the US, so the brands might be different on your side of the pond (I assume you're in the UK, since you called the cat a "moggy.")
Also, sprinklers with motion detectors that hook up to a hose and turn on when a critter breaks the beam. Point it in a direction where you won't trigger it, though. And the sprinkler things are not so great in drought-stricken areas, because the junction between the hose and the sprinkler leaks a bit. I tried one to stop my dogs from fence fighting with a neighbor dog, but unfortunately, it turned that corner of the yard into a muddy mess (and even without a drought, this is an issue with dogs tracking it everywhere). There might be better brands. Mine was called "Scarecrow."
I believe there are speakers that play high-frequency sounds that repel animals (the sounds are out of the range of human hearing). Someone in our neighborhood had a rookery tree, where egrets were nesting en masse each spring and summer. The birds were lovely, but they were also messy, noisy and smelly, and the homeowner and their neighbors got tired of guano, fish, and dead baby birds littering their yards and parked cars. When the person mounted one of those speakers in their tree, the birds never returned. I imagine it might work to keep cats out of the yard too, though if you have dogs or cats of your own, or enjoy watching birds and wildlife in your yard, the sound might bug them.
Here's
a list of some "cat repellent" products.
There may be some good advice here too:
http://www.alleycat.org/resources/how-to-live-with-cats-in-your-neighborhood/