I've linked to this before when people have had the same problem...apparently it's very effective, though I can't claim to have tried it myself yet (am getting round to it, prevented only by inertia! )
You could maybe grow some in pots and then move them around as needed? These aren't the only people to stock them; if you google you'll probably find more.
Aaarghh, I have a cat! That's how I knew we'd had cats in our garden over night, she started curiosly digging uo their toilets! So, I can't really get a frequency cat scaring thing cos then my kitty can't play out.
Again, with the water pistol cat stalking idea, i'm not overly keen on teaching my dcs to terrorize cats as they will quickly start terrorizing ours!
I have bought some of that garlic sprinkle stuff from B&Q but I will definitely have a look at those scardely cat plants, thanks Terps, as the garlic powder is only good till it rains.