I think it's a bit simplistic to say you should simply knock the price of a cambelt off the asking price. I think a 2.5 V6 with 48K miles on it, in good condition, of age 2002 or newer, has got to be worth £2000. If it was on a garage forecourt in Bristol I doubt it would be much under £3000.
Things I would be more concerned about are:
a) Does it actually have a service history showing 48K miles?
b) Does it drive like a 48K mile car? (i.e. feel fairly new)
c) Is the clutch heavy?
d) Given that it would have been made circa 2002, is there any rust in the floorpan?
e) Are you sure you want an auto? I personally wouldn't.