Insurance valuation

I've tried without success to get agreed value insurance on our 2006 Defender, problem is it's too new for this type of policy.
I'm concerned that if it was ever stolen there would be arguments over the value. I've heard stories of people being offered a fraction of the value after a total loss. The Defender 90 CSW in mint condition (ours is like new) with low mileage seem to go for £15-17k in the classifieds.
How do insurance companies value the Defender? It's not in the car guides (Glass's/Parkers) so do they look at selling prices in the classifieds to offer you a payout?
Last months Land Rover Owner had a buyers guide on the Defender, the featured one could have been ours, same colour, same mileage, same year....it's up for £16,995
Would the insurance company take this kind of thing into account? We are with Direct Line but none of the companies I called at renewal would do an "agreed value" policy.

Defender is in the price guides as a commercial van rather than in the car sections

I use heriatage insurance. I have an agreed valuation of 10k for my 1985 defender.... Although it has had loads of money spent on it, i just sent in 6 photos and i'm on a 5000 mile limited policy per year.

Not sure about defenders, but When my Navara was written off the vaulation, was worked out over the phone on autotrader, they just looked at what others where selling for, and paid me that.

Try NFU my 1996 110 is valued with them at £11,000 gave them loads of photos and a long list of what had been added to the vehicle.