I was told to expect no more than 3 or 4% off when I bought a new car for someone else for the new 12 plate. But I ended up with more than 10% off, on a car with a four month lead time.
I just went to three different dealers and asked for their best price. I didn't waste their time with test drives or trade ins, just printed off my desired spec and walked in with it. They all say that they don't want to get into a dutch auction, but they can't stop themselves getting drawn in and undercutting each other.
Top tip - keep it very friendly. If they like you, they'll try a bit harder for you. Be patient, and when they eventually do pull out, you know it's because they're no longer making enough profit. Then sign quickly, before they change their mind!