If they got a judgement against you and you didn't know about it, you can have it
set aside. This is actually quite straight forward and I've been told by a Sheriff before that in practice, they almost always grant an an application to have a case
set aside (technically called a 'Minute Of Recall), if there is any question that the defender wasn't aware of it. HOWEVER, if they can prove that they sent you a copy of the original decree, then you're probably screwed.
Natalie is probably right when she says they can't just take the car, but on the other hand, if they can show that you haven't honoured your end of the deal they might be able to. You should contact your local Citizens Advice Beurau for guidance.
You should absolutely go back and do a Data Protection Act request on the lender to see how much you've been charged, because you'll need to deduct this plus
interest from the final bill.
Good luck, and don't forget to keep us updated.