Again - it's on dodgy ground but it didn't happen to me so I didn't read up on it in depth. I think the view though is to reluctantly pay them so you get the use of your car back (don't attempt to take it off because that would be criminal damage) and then persue the company through the Small Claims Court for the money back.
There's something in case law (contracts) about the cost being proportional to the contract and in this case I don't believe it is - another point in your favour.