The legal route is to apply for a court order for the return of the car under the Torts (Interference with Goods) Act 1977. In practice just threatening to take legal action might do the trick:
"Sir, I demand the immediate return of my vehicle, with the repair that I have already paid for completed. Failure to return my vehicle to me forthwith will result in legal action under the Torts (Interference with Goods) Act 1977, together with a claim against your company for payment of my legal costs".
Alternatively (and possibly better) simply phone your local newspaper and tell them about it. It's amazing what a bit of press coverage can do in such cases!