The only thing I've found that works with car dealers is to be straight to the point, don't fanny around, and above all. Don't look like you give a [censored], if you don't like the price they offer, say you'll look elsewhere. They'll either [censored] themselves and knock the price down, or say no. In which case, walk away. The best plan of attack is to use another dealer as an example, I know of someone who got several grand knocked of a new car by simply not giving a [censored]. Go to parkers.co.uk and get the book price for the car and work from there.

Being able to pay the full amount with a draft will probably make the dealer more willing to do a deal.