Thanks for all the helpful advice.
Agreed on the part-exchange usually being another opportunity for a car dealer to rip you off, but the price they gave me for my X5 seemed pretty good, enough to merit avoiding the hassle of selling it privately anyway.
I saw the broadspeed site but it doesn't allow for extras which is what made drivethedeal so handy. I remember getting a decent discount on a 535 using that...
So it sounds like I should aim for 8% which is around £4k - which is a long way from the zero room of his initial position. I can just walk away and do nothing, so should be interesting.