I would say its a bit over priced, fortunately with the end of a quarter/month coming you are buying at a good time to do some bargaining, offer them £18k and see what they say, be prepared to walk if necessary and let them sweat.
They'll possibly say they have other parties interested, but just stick to your offer and say they should let you know if things dont work out with the other parties.
Hardly anyone is going to buy a car like this before Christmas so it will be easy to screw them a little,
R