If you signed papers for the agreed amount, then as far as I know, they haven't got a leg to stand on. Tell them to swivel. 
You have signed and paid the invoice amount, their mistake - not yours.
Andrew
.... x2
Except that I'd be polite and charming and say sorry but it's not your problem and you can't afford to pay for their mistake. No point in being rude to them - You never know when you might want a favour or help in the future.
Robin's got it on the head. If you bought a loaf of bread & some beans at Tesco and were undercharged, the management have no redress in contacting you to say you underpaid. I certainly wouldn't come over with a story of not being able to afford it as the previous post advises. Be firm, but be polite, with an air of knowing where you stand. If you aren't sure, your local Trading Standards dept are very good at this sort of thing. Even better is
www.honestjohn.co.uk the champion of car consumers' rights. You can expect a swift email reply.
If you are in posession of the correct paperwork, including a receipt, then there's precious little the dealer can do. It only strengthens your position if the salesman & his manager have both dealt with the paperwork. Although I'm disinclined to believe the salesman's "I made a mistake with the price" sob story, even if he did, you cannot be held responsible for that in any way. Just explain to them that ,as they know, much of the profit in cars these days is in the hearty labour charge & warranty work that dealers carry out. As a loyal customer, you'll be happy to patronise their parts/service dept, but you certainly won't if they want another £300 off you.
Bottom line, like Hammer said,
they can't touch this
