I have the same problem, I have a 53 is200 with 45000 miles and aero pack 2 and was offered £5000 but managed to push him to £5500.
it was a 07 is250se auto 9000 miles I looked at for £18495, nice car but no chance!
tried quite a few garages and it seems thats all my car is worth, but good luck and hope you find a deal that you are happy with, don't think I will.