Cat Burglar?

How do I stand with regard to a neighbour´s cat which has adopted us?

Our female cat brought a long-haired tom from the next street into our garden 4 years ago. It stayed longer with each visit until we thought we should try to find its owner. We put a collar on with a "contact us" message and the owner rang soon after. He came and collected his cat several times and we delivered him back home too, but it was clear that he much preferred our home and he has stayed with us ever since. During one visit the owner became very abusive and threatened to hit me for stealing his cat. Shortly after this the cat turned up with his fur shaved to the skin all along his back from head to base of tail. We suspected the owner had done this for some reason. We have sheltered, fed, and looked after the cat´s medical needs ever since, and he stayed on when our own cat died.

Last week he disappeared for 7 days but turned up last night with a collar showing his name and a phone number which I think is his owner. He made a huge fuss to come in and shows every sign of staying, whatever his owner, who we have not spoken to, may want.

I´ve read a lot about strays being adopted, but what the law say about cats who choose not to live with their legal owners?

Am I stealing a cat by opening my door to it, and does the owner have any right to insist that I don´t shelter or feed it? There´s no doubt at all about where the cat wants to live and I would have a real fight on my hands if I tried to keep him out!