Richard,
the valuation is based on what I'd have to pat to buy a replacement with the same level of trim and accessories, so £5k above the price in the market is reasonable.
Mine is a well documented factory car fitted with every accessory on offer at the time, including AeroMax wheels, it was the first Aero of any description with paddle shift for the auto box.
I've upgraded the audio system to DAB, fitted MMC specification LED headlights, an Aero Racing roll bar and SSL suspension.
I really hope I never have to claim, but as John found out when WVM hit is roadster, there are idiots about!