Dealing with this as a trading standards/SOGA issue isn’t working. Also, rather sensibly, you are aware of the dangers of owning with bombers.
I’d be turning up, with witnesses, during working ours, with all you paperwork, then phoning the police as a robbery is in progress. when they arrive, explain that he has your money, you have not authorised repairs to your vehicle, there is no reason why your vehicle is in bits in his workshop unless he plans on breaking it for sale.
Until he gives you a full refund, the car is yours. I doubt he can come up with an excuse to the police as to why he is in the process of breaking your vehicle.
You vehicle removed from site and held as evidence, police crawling over the business, phone your insurer as the vehicle is stolen.
police not interested in a civil case, but if he’s in the process of stealing your vehicle…