The answer lies in the Tort (Interference with Goods) Act 1977, sections 12 and 13 and the Schedule to it apply .
You are what the Act calls the "bailee" of the vehicle. The customer is the "bailor"
What the Act says, basically, is that you,the bailee, give notice to the customer,the bailor, that you intend to sell the goods but if you cannot find or trace the customer then you can sell them anyway. Having sold them you are supposed to hold the proceeds to the account of the customer less any costs you have. However, it appears plain that you can go to the County Court and get an order that you sell the vehicle and that you can ask the Court to further order that you deduct ,from any proceeds you may get, the costs of storage and of the work you did on the vehicle.
Your local CAB or the County Court itself shoud be able to guide you on this Tell them of the Act, so they know instantly where to look , how it applies to you and what you are trying to do re work done .