I've posted on here a few times over the last three weeks because our two young cats seem to be falling foul of local thug-cats. First, the bigger boy came in with a large roll of skin flapping on his back paw. TBH it didn't look like a bite (shallow, no puncture, quite clean edges) but given what's happened since I'm now wondering if it was a very sharp claw tear.
We took him to the vet, kept him in nearly two weeks to give paw a chance to heal. That was about £260 all in but more importantly, he was depressed at being indoors and desperate to get outside! Windows etc couldn't be opened during that time as he forces them.
Two days after we let Big Cat out, Small Cat comes back in with a very sore and limp tail. 
They are brothers/littermates but Small Cat is about 2.5 kg lighter and tiny! For a while we worried his tail was broken and would have to go, but fortunately, once the vet had sedated him to have a look/x-ray, it was found to be a deep bite and he went on antibiotics and loxicom for the pain/swelling. It abscessed initially and I had to poultice it to get the pus out. Small Cat is still in but wants out, obviously. Meanwhile, Big Cat came up the drive yesterday holding his tail suspiciously low and yes, he's also been got at! Luckily not a nasty deep wound but a very shallow scrape and a bit of fur pulled out. I dosed him up with loxicom (luckily I bought the BIG bottle from the vet) and washed the graze with salt water - I'm getting to be a pro at this...
Fortunately Big Cat is fine in terms of his wound. They both want desperately to go out. We live at the edge of a village and there are large gardens they roam, plus a big meadow at the back of the houses. The house is set back from the road into the village and was deemed pretty ok by the cattery and by our cat-sitter - basically it's about as good as it'll get, safety-wise. I'm not of the 'Keep cats indoors/catio' camp, and I've always said that cats need to explore and roam freely in order to be cats, and that comes with some risk ...
... but now it's right in our faces, I'm feeling very conflicted. The suspects, Bad-cat 1 and Bad-cat 2, live two gardens away and moved in with their people nearly 2 years ago. Unfortunately, though we've been here 15 years and had a cat for most of that, the new cats moved in during the brief 6 month period we were without a cat and therefore presumably the 'bad cats' see themselves as in possession of the territory! Having said that, we've let our cats out since late last year and there have never been any injuries until about 3 weeks ago. They've had cat stand-offs until now where they Sit and Look at one another and occasionally chase, and that's been it.
I need to go and talk to the owners (I don't know them at all), but very gently and politely. And I'm not even sure what I'm asking. I'm not asking them for vet fees obviously, it's our own choice and risk to let our cats out. I'm not asking them to keep their own cats in, even though their cats appear to be the aggressors or at least are coming off better (this is something I thought I'd ask them though). I suppose, if it were my cats who were constantly stomping on someone else's and causing injuries/vet's bills, I'd want to know.
What helpful things can I suggest to them? I thought I'd ask if anything has changed, because their cats' behaviour has. Should I offer to get them a super-soaker water pistol to squirt at the cats if they're fighting? Ask if they'd like me to get them a Flyway plug-in to chill them at home?? I can't think of anything else! I'm thinking I'll get a super-soaker myself to squirt the buggers if they come into the garden, it's just harmless water and maybe they'll associate approaching our cats with getting wet.
I'm sorry for the essay, I don't appear to be capable of brevity when I'm anxious, but how have other outdoor cat owners dealt with aggressive neighbour cats?