If your ex partner is the registered keeper then it could be difficult but not impossible for you to demand it back. The fact that they are the registered keeper of the car does not mean they have ownership of the vehicle, it gives them no legal title to it whatsoever. In order to retrieve the vehicle you would need to establish ownership and ask, in writing, for it's return. This is of course only going to be the case if the finance agreement is in your name.

It would be wise to check your finance agreement for the vehicle. If it is Hire Purchase then, technically, neither you nor you ex partner own the vehicle. It still belongs to the HP company. It is on Hire until it is Purchased, this usually takes effect on the last payment to the agreement, often called an Option to Purchase fee. If the agreement is in your name call them and speak to them about the situation.

If you cease making payments to the HP company then it will be repossessed by them. This could be automatic right on default of the agreement or by court order, depending on how much of the agreement has been paid. Perhaps advising your ex partner that you will do so if the vehicle is not returned will be sufficient to persuade them to return it.