I can supply still warm poo at £5 a lump. You'll need to supply sundries; a Tika curry, beer and a copy of Event Horizon.
I'd try one of those sonic repellers first. Get a decent one if it's a fairly big garden (so it's loud), and position it / point it close to and at the place where the cat is coming into the garden.
We've had five cats at one point, and they can only get into the garden by climbing down the wall or tree, because the walls are too high for even a kitty to jump off. So, in my garden, I'd stick the generator right by those.
If it does nothing over a week, take it back and get a refund (this will be much easier if you buy it from a big chain store like Tescos, were the £10 means nothing as they haul in tens of thousands a day per store).
Cats aren't interested in their own reflection, or pictures of cats.
But, I found a funny looking plastic cat at a garden centre one day, with neon green eyes and a face on it that actually upsets quite a lot of people (looks like the cheshire cat). Surprisingly, I put it down in front of one of the moggies facing them, and it jumped and started walking backwards.
Make sure there is no catnip in the garden, they will also be magnetically attracted to running water, thinking a fish might appear if they wait for a few hours.
My neighbour has a pond and gets annoyed by the cats. They'd run up there, catch a frog, bring it back, we'd catch the unharmed frog, carry it back up there and the cat would be trying to weave back out of the house to get another in the meantime.
He had a water fire extinguisher and would squirt the cats from the back door, then charge it back up with a foot pump. Having a garden hose with a spray gun right next to the window would help.
If possible, wait for them to get half way through laying one down so it's harder for them to run off as you open the door or window.
Getting rid of catnip isn't too much of a problem, but your pond or water feature would be (unless you don't like it anyway). Spraying them takes effort, as they'll sneak in when you're not there.
The scent based methods do tend to wash off, or just get ignored. Even if you have a dog, the cat will sit on the wall and wait for the dog to go in.
They're pretty darn tricky to get rid of if they're intent on getting in there. Which leaves the sonic repeller or trying a hose.
If you go the hose route, really get on top of it for a few weeks and try to soak them (you have to get cat like cunning, and wait for them to get into a situation they can't easily run away from - which they don't often do).
Alternatively, if there's only one or two ways in, you can think of novel ways of physically blocking them. Pins on the walls don't work so well, as they can step over them.
Oh... most cats also hate anything that looks or sounds like a snake. Even the shape of the hose (if it's moving) will scare them. If it makes a hiss or rattle sound, they'll run away too. E.g. if I open a bin bag, get out the tin foil or put some deodorant on, they think most of those are a snake in a can.
As a reference, there's an episode of Black Books in series 3 where the owner of the building Bernard's shop is in dies, and it's past on to his cat. The cat then increases the rent. Bernard gets so sick of it, he tries paying a rentokill guy £1k to 'do it in', saying he'll stick a knife in it's back afterwards, hide in the library and say it was professor plum.
