Negotiating the Best Price - Used Honda Civic Tourer

[Skirmisheer]

I'm currently getting very close to buying a used Honda Civic Tourer after doing the initial searching on Autotrader. The prices of the cars I'm interested in are typically around the £14k - 15k figure and the cars at main Honda dealers.

It would be useful if someone can give an idea of what figures I should be offering. Is there a percentage to knock off the asking price or is it more complicated and involved than that?

Any advice on my future purchase would be much appreciated.