My Cat Is Marked As Missing By Her Original Owners!

I rehomed a cat and her kitten in October 2024, from a young girl and her parents who couldn't keep them/didn't want them. The father practically chucked her in the carrier and we barely got in the front door. I have conversations asking about them and her giving me her address and number as well as the cats name.

Fast forward to today I took her and her kitten to be neutered and microchipped and collected them about an hour ago. They gave me the kittens chip details but said the mum was already microchipped. I asked for the number they searched her number and she came up as missing!! They called the number on the chip and left a voicemail and sent us on our way. I just got a call when I got home saying the vet has said we need to bring her back as the owner wants her back! Obviously if she was marked as missing they should've checked this before spaying her but they didn't check!

I just sent over all communication which clearly shows she was rehomed to me and the nurse is passing it to the vet. I contacted the girl I got her from who has said the details are her brothers on the chip and he was fully aware they were rehoming her as they couldn't care for her properly and it makes no sense as he's not even allowed a cat where he currently is.

Where do I stand legally in this? I'm obviously NOT taking her back to the vets. I'm in the UK.