Hi matt,
I've also seen the ad for the 250, You oviously read the Trade it! Don't worry I'm not interesred in it for myself I just like to keep an eye on the secondhand Naim market in my area!
I agree with Alex, it seems like a very good price, £800 would be more like it for one that age! Like he says though, check with naim for its history quoting serial number, as for estimating a realistic sell price for your old 180 I,m guessing you could easily get at least as much as the sell price of the 250...A 10 year old 180 recently serviced by naim in good condition should easily fetch £600.
PS serial numbers & their corrosponding manufactured dates are available on Naims site ...have a look for it!
The amount of sonic benifit of going from a 180 to a 250 will in my experience depend on the speakers it is driving....For example I tried a 250 to replace my 180 with my Epos ES/12's expecting a definate improvement ...in short it didn't sound improved at all ...the 180 was faster punchier clearer ...more fun!
I thought the extra juice would help to drive the speakers better ..more control over the drivers!
I was wrong, however before you get to worried let me tell you I have heard the diference a 250 can make to a pair of Alleas in my set up..much better then with a 180 i can tell you!!!
cheers for now

Matt

Mad Boy Matt