keep the cash, pay it back in very small amounts per month! as r32uk said! 
As I said, come to a reasonable arrangement with them... don't try to shaft them. If they think that you are taking the mickey they will drag you through court and the courts will make you pay a reasonable amount... and probably the costs... and then there's the costs of your own legal representation. Offer to clear the debt in a reasonable timeframe.
Yes, the dealership made a mistake but pitman4u still has their money. How would you feel if they had overcharged you and then they had said, "We'll pay you back, £5 per month."? I know... you'd be all over this forum complaining about how unfair it was.
Pitman4u still hasn't explained how he didn't realise at the time that he had been undercharged by £1000 - I'm damn sure I'd have noticed...
It has been pointed out to me by a solicitor friend of mine that, in law, the ownership of the vehicle might not have transferred (and before you start bleating on, the V5 does not prove ownership, only who the registered keeper is) as the contract to purchase the vehicle has not been completed; the agreed purchase price has not been paid... i.e. there is a good chance that in law the dealership still own the car.
You can take whatever action you like, I guess, but you need to be aware of the potential consequences...