My story:
Someone drove into my iS 2 months ago, insurance valued it a £700 and tried to Cat C it (Based of Glasses Guide value for an E36

). I went at them with a two pronged approach:
1. I knew their valuation was as good as made up so I asked, in writing, for evidence of where they got their valuation from, knowing they were in a corner.
2. With the help of Zoners I buried them under other ads for iS's, and picked holes in each one explaining how mine was better. I also scanned my BMW Service Book and all the receipts that came with the car (totalling over 10k in the past 22 years

) and sent them some very shiny pictures I'd taken of the car just a week beforehand.
Then I waited. If you don't get a response within a week or so ask to speak to the claims handlers manager. If they don't let you do that then ask them for their formal complaints procedure (That tends to get them squirming a bit....)
Then, about 4 weeks into my claim, I received a phone call. The Claims Manager had reviewed my case, and the car had shot up from £700 to significantly more than the £2000 the garage quoted for the work overnight!
My iS is currently being repaired (It was two months last Friday someone drove into it

)
But the moral of the story is this: You can win by being proactive - get a freephone number and call them every day, demanding updates, not being afraid to call BS on their part, having a figure you want in you head and not being willing to settle for a penny less and just generally being a pain in their backside!