You should only pay the price stated in the car window, or ask the salesman for the price on the car, then normally knock £2500 off that price negotiation.
I rarely pay full price for anything, my best time to get a deal is buying a car, or fairground rides.
Remember, your walking into the garage to buy 'there' car, therefore you can leave at any time.
Take a test drive in the car, mention that you have seen 'others' in car garages down the road, then after the test drive, look round it. Pick out all 5the work you want done to the car, ie wheels replaced, chips, marks etc removed. Then get to work on the price, if the car is up for £18,000, tell them you want to pay £14,000 and go from there, then when you feel the price is not coming down enough, be prepared to leave, tell him your best price and write your number down on a piece of paper, give it him and say 'ring me if the price changes' then walk away, i can assure you that by the time you get too your car, the price would of dropped a bit more and you be ok for a deal.
People make the mistake of sitting and excepting the price. I like to hammer them down, then walk away for a better deal. They think your going to go too the other car garage, if the car has been sat on the forecourt for a few weeks.
Thats how i got my car from £13,500 down too £10,600.