How can I prove ownership of a car registered, to a relative?

Bought the car in sep last year and have the original bill of sale, registered the car in a relatives name jan this year on the agreement that she would pay instalments. She didn't through no fault of her own, she had to leave the country in feb this year. Car was impounded for being untaxed last week. Met now won't give the car back as the v5 is still in her name and the daily charges are mounting up. Its a 4 year old car and cost me a lot. Whats my recourse?