Thank you all
Someone did suggest the noise thingy to me before, it's called a Mosquito, but they're like £500 plus installation. I would pay it, but knowing my luck it wouldn't work on the little sods.
I dogsit for my friend once a week but her dog is useless

, he doesn't bark and won't play with balls

My son's girlfriend also has a dog, he does bark but will only play with tennis balls so not much use either!
Initially I always threw the balls back when they knocked, then they stopped knocking and just started helping themselves. When my eldest son is here he's booted quite a few from the back of our garden, there's about 4 different gardens that back onto ours so no idea where those balls went. We also collected up a load last summer (about 10 in a fortnight) and gave them to a local kids club. And still it continues! I have periodically been throwing some back over but I'm done with doing that now.
One of the kids plays for the youth team of a Premier league club (you'd think his aim with a football would be better in the circs!) and they basically treat him like the Second Coming. I don't know if they get the balls free from the club but any any time they must have 10+ footballs in their garden (or in mine).