To answer your specific question, no you cannot legally dispose of the car.
You have the right of "special lien" over the car which gives you the "passive right to retain" the car until the specific (but not other) debt is paid.
If the value of the vehicle exceeds the debt you can plead to a court for an Order to sell the car and deduct what is due to you from the proceeds. To be able to add storage costs you must notify the owner of your intention to do so and state the daily rate (but you cannot make storage retrospective for the past year, however). Similarly, if your trading terms do not include a statement regarding interest you can charge interest from a future date but not retrospectively. Or you can notify the owner of your intention to return the vehicle to him, state the cost, then issue proceedings for all of what is due. But do not sell the car, otherwise it may be you who will receive proceedings !!