I did this with my insurance its takes ages!!!! Basically my advice would be except the money they give and then you argue for the extra on top. Basically don't use glass or parkers go on autotrader and find loads of trucks like yours (Age, Mileage, Spec) then find the most expensive ones (not private). Make a small portfolio of trucks you could buy as replacement. A recent photo of your trucjk showing spec and condition will help (be careful not to show performance mods that would effect your insurance). Plus copies of receipts for recent work and if you could get a testiment from your local garage of the condition of your land rover being above average that willl help. I did all of that and got 1500 extra for 300tdi 90 which was stolen but I took them 12 months to decide.
Hope thats helps any more questions pm me.
Dave