Car left in my drive as collateral against debts owed. Now owner wants back without paying and police agree.?

The car has been in my drive for 5 months and no longer taxed. Didnt hear anything from owner till police called to say reported stolen. I explained about debts but was told as no written agreement tough. Is there anything i can do. He has money and another car. Im dissabled and on benefit so i cant afford to take him to court. Although police say its a civil matter they still insist i have to let him take car. I have 3 indipendant people he told the car was being left to cover debts but police not interested.