This is a bit long but I'm a bit upset and so it's hard to be concise! Our flat has access to a very large communal garden. Our cat is elderly, very well behaved and goes out for a little patrol or a snooze in the bushes a few times a day. Usually this is supervised. The only exception is in the mornings my partner will feed her and open the back door on her way to work so the cat can go out if she wants before I wake up. There is a dog who lives upstairs and before we moved in used to be let out unsupervised. They kindly stopped this because of our cat.
Anyway cat was out in the morning before I woke up. The neighbour didn't see her so let the dog out which then chased the cat out of the bushes and went for her, before grabbing her leg and dragging her down the tree she was trying to escape up. The owner at this point pulled the dog off and shooed the cat back inside.
I (being asleep) knew absolutely none of this and so woke up, heard her bell jingle under the bed (where she often spends several hours napping or hanging out with a toy) and carried on with the day. In the evening I thought something was off so tried coaxing her out with food and treats but nothing- we had to pull the whole bed apart in the end to find that she couldn't get out because her back leg had gone limp. At this point our vet had closed and so we took care of her until we could
rush her in first thing. With her age and mysterious onset the vet had to run every test to try and identify the cause (it could have been a clot or something neurological). A day and a £550 bill later the vet found two of her lower vertebrae to be quite crushed (worsened by arthritis) and severe bruising to her leg.
Two DAYS later the owner of the dog has come round and told us what happened, after another neighbour told them our cat had been mysteriously injured. Apparently at the time they tried to say but they left it because 'nobody was home' (despite the back door being open 
).
I am angry about what happened because of the pain my lovely cat is in, but I realise it was an honest mistake, it's a dogs nature and with sharing a garden accidents happen. I am however LIVID that they left it almost three days to mention it, and only because someone else guilted them into it. Not only could I have taken the poor cat to the vet much earlier and gotten her out of pain quicker, we could have halved the vet bill by not having to explore every avenue. I just feel that if it had been reversed and it was my dog, I would have been round a few times that morning at least to try and let the owners know asap. AIBU? Some people I've told agree with us, but others have said we should be grateful they said anything at all. I don't know what to think and I'm up anyway so I thought I'd ask what you think.
tl;dr: neighbour's dog attacked my cat and then they took two days to tell us