What is the value of the cat?
Why was the given to you by your partner’s parents rather than your partner or both of you?
Why did your partner, suddenly, 2 years ago decide to start paying something towards the vet bills?
The name on the chip may or may not be the owner. In the same way that a motorcar V5 is not proof of ownership, the chip details are just contact details if the animal is found.
You are correct, the cat is not hers, the cat is yours because you never transferred ownership of the cat to her.
However if you go to the police, because she takes the cat, the police are not likely to get involved because of the issue with the chip details.
If she takes the cat, you can apply to court for an injunction (and as the court for court and legal costs) to make her return the cat to you.
If she simply takes the cat, by all means report it to the police as theft even though there are unlikely to do anything because of this chip issue.
Ultimately, if she takes the cat when you are not there, you are faced with making a court application.
My suggestion is to tell her that if she insists on taking the cat, you will make an emergency application to court to prevent her pending a hearing over ownership of the cat. You have plenty of witnesses who will attest to the Being yours and all she will have a few vets bills which he paid.
On the balance of probabilities, the judge would find the pet belongs to you based upon the facts you have given.
Can I clarify anything else for you?
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We can still exchange emails. Best wishes. FES.
You can go to court to get an injunction/interdict either to prevent her taking the cat or to make her return it but it would only be a temporary remedy pending a full hearing to decide ownership.
If the person taking the cat decides to defend the action it could easily cost £5000 or even more to the loser. That would just be legal costs of the losing party.
It’s likely that just the solicitors letter threatening the court application and substantial legal costs is likely to make her think twice about taking the cat.
That is what I would do at this stage.