If they still have your tortoise. You could try speaking to Maggie Coleman username "boxgirl" on here. At the health checks she organises she has been taking ID photographs of torts for many years now. Both carapace & plastron. To prove ownership in a situation like this. I believe every torts plastron is unique & stays very similar (if not the same, cant remember now) over a long while. I have her contact details somewhere if you would like to ask her advise ( im sure she will be happy to help you) pm me. Good luck I really hope you get your tor back