Let me answer this as far as I am able.
No 2 lawyers have the same way of dealing with something.
My personal view here is that I would pay the bill, get the car back, and then take them to the Small Claims Court (assuming the claim is under GBP10,000) on the basis that you paid under pressure/duress/mistake, to get the vehicle back.
It depends whether cash flow will allow it because a small claim, if defended, could easily take 12 months to get to court.
The court fee alone for an injunction application is GBP308 and there is a lot more work involved with the application and the witness statement and certificate of urgency and it may not work because of the alleged debt hanging over you and the vehicle.
You could use the injunction application form N16A which would also need a witness statement
The majority of court documents take the same format. Here is the format of a Witness Statement
Please note that the Statement of Truth at the end of a witness statement has changed and here is the new wording: https://parissmith.co.uk/blog/guidance-signing-statement-of-truth/
So you will see that the payment in small claims is far more straightforward.
Thank you for letting me assist you with your legal question. I am glad that I was able to help.
I am not certain whether that answers the question for you or not, but I am happy to answer any specific points arising from this.
It will be my pleasure to help you again either further with this or any future questions you have