If the finance is in your name, the original purchase in your name and you are poaying it via your bank account, then your ex is only the registered keeper as per log book and has no legal right to sell the car. In practice however that does not mean that he can't.
If you have the docs that prove you own the car you can fill in a V62 and Get a V5 in your own name then the one he has will be invalid you can then reclaim the car yourself, if he won't give it back tell the police it was taken without consent and they'll do him for Twocing.
Doesn't matter who has possession of the car or whose name the V5 is in, if it is on finance the bank own it and it can't be sold without the bank's permission.