Hi Martin. Aiden is correct. Your right of lien takes precedence but only on the current outstanding work. You gave up the right to withhold property pending payment when you released the car previously, you have to sue for earlier invoice. I had similar where car was owned by hire purchase company and customer disappeared owing £800. Company telephoned to arrange uplift, I explained no problem, on payment of invoice due. Company manager said that as they had not authorised any repair and that they owned the car, I had to release it to them. I responded in a negative manner and requested that when he had completed that task he should check with his lawyers. They did not show up.
I applied for registration document ( DVLA presumably wrote to address from which customer left) and was issued same. Used as spare car for three years then sold.
Hold out and don't flinch - Tom