It is very important that you keep a record of all communications. If you feel that you do not have an adequate written record, you should write putting all the details in a letter and send this by recorded delivery. In your shoes I would be inclined to let them have the car back (I would even offer in writing to deliver it to any local address just to make the point that I wasn't trying to run and hide) and then I would get ready to dispute any amounts that they wanted paying, in court if necessary.
When writing, you should state that, in the circumstances, the terms of the voluntary termination clause (please read it as these are not all the same) should stand and any charges that are not mentioned in this clause will be disputed.