I wrote my 182 off around xmas 2010. When the insurance company came back to me they where offering around £3,400 for a replacement.
I told them to jump. I asked how the calculated there figures. They did the same as you said, using trading sites and car guides.
They said if i do the same they will re-evaluate.
I had to send them 5 articles, used autotrader, pistonheads and cliosport and found examples with more miles going for more than my offer.
After about 3 weeks of chit chat they raised my offer to £4,800 which is was pleased with.
They also give me £70 for a full tank of super unleaded.
I know this might not be any help to you, but just hold to your guns. Its not like there going to say You where been an arse so we now only want to offer you £4,000.
Hopefully it will pay off and you will get it done and dusted.