you can also try here for info on buying online.
http://www.which.co.uk/advice/buying-cars-online/checklist-and-contacts/index.jsp
that's how i managed my 10%, I went to see the dealer and then got prices from brokers4cars and autobytel (these are part of inchape who own Audi dealers in south west England.) they will quote about 8% off list. the trick is to get them to agree to this price, give you dealer a call and tell him what you can get it for, they will match it, (if not walk away and find another dealer). if they will match it, arange to go an see the dealer and the 'buisness manager' and then start to hit them harder and negotioate, if they won't give 10% walk away and find another. it took me a couple of weeks but in the end i had 3 delaers that were willing to go to 10%.
one thing to check is that you dealer are part of different groups if you are going to play them against each other.
good luck