There's a considerable difference when a vehicle is taken between "Stolen" and "Taken Without Consent".
The police are unlikely to be interested in an accusation that it was stolen. To prove that they must show that it was taken without your permission and that he intended to permanently deprive you of it.
Taking without Consent ("TWOCing" as it's known in the trade), is a slightly different matter, though still covered by the Theft Act. As far as TWOCing goes "...a person shall be guilty of an offence if, without having the consent of the owner or other lawful authority, he takes any conveyance for his own or another’s use..."
Your problem is that he had your consent when he took the vehicle but that consent was subsequently withdrawn. I suggest you hang on for a day or two to see how it pans out and when it is clear that he has ignored the agreement you had come to for him to return the vehicle, visit your local police station. Keep any evidence you have that he was asked to return the vehicle and that he acknowledged that request.