You have three issues here. Firstly the car, secondly the charges and thirdly the tribunal.
Firstly the car. Even if you owe him millions, he has no legal right to keep your property. It is called theft. Write to him (Keep copy of letter) and state that you wish to have your property back. Set a time limit. After this period, take legal action.
The storage: He has stolen your car. He is then trying to charge you. He is, in effect, making money from a crime. In your letter, state that you do not agree to these charges. Also, state that unless car returned by the deadline mentioned, you will charge him for rental, £1500 per day should do.
Tribunal: Keep records of all conversations and correspondence. Take to tribunal. I would like to see how he fares following attempted blackmail.
Finnally, get legal advice. I am not a lawyer and am giving you advice as to how I would handle it. A lawyer could probably make him go away a lot quicker. If you are going to a tribunal, you are probably (If smart.) using a lawyer. Go to Him/Her in first instance.