corgi has a good point - I dont think he's taking it too far at all TBH.
At the end of they day, you agreed to purchase a car for £13,400. That was all fully agreed by both parties.
Your receipts indicate you have paid £12,400.
If you even took this to court, do you really expect them to say its VW's fault and waive the last £1000??
At the end of the day its money outstanding, and it will need to be paid.
What I would do is state that its their mistake and that you want interest free finance or somet to pay the remaining balance back since you no longer have the funds. They are not obliged to do this, but may consider it out of goodwill. If not, you'll have to pay the £1000 up front.
The previous company i worked for had some employee's leave. The company continued to pay them for 6 months after they had left!! Obviously these leavers just pocketed the money and kept quiet. Nevertheless, when the error was highlighted at the end of financial year, the leavers still had to pay back every penny - even though they paid back small amounts over a longer period of time - fact is it was not their money so they had to pay it back.
Same here - that last £1000 is not your money, whether VW claimed it after 5 weeks or 5 months. You could take it to court, but I pretty much guarantee you wouldnt win any case.
Sorry mate, I dont mean to put a dampner on things and it would be great if you could pocket it, but it just wont happen.