If the car is under a hire-purchase contract then the hire -purchase company has legal ownership of the vehicle until the loan is paid out.
If the agreement is in his name then the ownership rights are between himself and the company, unless a separate agreement is in place to assign ownership rights to yourself.
This usually involves approval from the finance company as they don't want to have more than one party on the contract.
If However you are both signatories to the HP contract, then you have equal rights of ownership and need not hand over the car. A visit to your lawyer would begin the separation process and you should seek an injunction on assets until a hearing decides the outcome. Ultimately it is a family court or civil matter, Good Luck.