from the view of the customer you cant legally withhold their car unless theyre refusing to pay anything. should the customer be offering payment for some of the work you have to give it back and as above it then becomes a civil matter for you to persue them through the courts for outstanding money rather than a police matter .
the customer could call the police themselves and ask them to attend as theyre offering to pay something but you are refusing to release their car and are acting in a threatening manner.the police will side with the customer and return the car.
thats what trading standards told me . whether its different in scotland i dont know.