I'd say it's not so much about how much is knocked off, but what it's worth. Asking prices are irrelevant really. Offer well below what Parkers and Glass's say it's worth, regardless of the asking price. Take printouts of any valuations you get (in your pocket, to bring out at the right moment), provided they're low.
The best advice I ever heard (before we bought our house) was...
If your first offer doesn't embarrass you then it's too high.
My other half runs away and hides when I start bartering. Also remember that car dealers could win oscars with their acting. I sold a car to a dealer for a very good price after he'd said he wasn't at all interested!
And leave your number and walk away if he won't play. He will ring if your offer is reasonable for the age and condition.
---------- Post added at 19:20 ---------- Previous post was at 19:12 ----------
If it's a 5-door then, from Parker's:
Original Price£12,755
Franchised Dealer£9,185
Independent Dealer£9,020
Private Good £8,610
Private Poor £7,215
Part Exchange£7,955
So I'd say the asking price is pretty keen already. But still well worth starting at £6k.
I'd suggest that, rather than arguing about its value (you can't), just say that you definitely want it but £6k is all you can afford. Don't worry about increasing this after, car dealers are not offended by fibbing - it's what they do all day.
And use the "power of the silence". If you offer and there's an uncomfortable silence just wait for him to speak first.