Having wrote my A5 (3.0 TDI S-Line) last Tuesday and going through the valuation / settlement process I would suggest :
Spend some time trawling the usual car sales websites but also specialist forum sales threads
Save links for cars of same model, trim, age / plate and miles (although ~15% leeway on milage should be ok)
See what Parkers, Glass Guide etc book price is
Don't accept their first offer if it's not what you consider (and can demonstrate) is a fair value
Ask to speak to the valuer and ask where their examples are from and advise you can provide your own
My valuer spent an hour on auto trader and said he couldn't find a comparable example so took a free valuation average of Parkers etc book price
I spent two hours trawling many sites and gave 5 examples of a higher value (3 were well off miles wise but the rest of the car was pretty spot on) two were "close enough"
They upped their valuation by over £1500
Good luck