According to the AA:
<<"Can a Garage Retain your Car?
A garage that has repaired your car has what is known as a 'lien'. This is the right to keep the car until payment has been made for the work done.
If you remove your car from the garage without authorisation you risk not only civil proceedings but also (except in Scotland and Northern Ireland) criminal prosecution.
The only alternative open to you, apart from legal proceedings or leaving your car while any dispute is sorted out, is to pay the amount demanded, marking the invoice as 'paid under protest'.>>
Some useful information here:
http://www.adviceguide.org.uk/englan...ge_repairs.htm
And here
http://www.adviceguide.org.uk/englan...ge_repairs.htm