Baylissboy - The reason is that insurance companies usually settle claims (other than agreed value policies) is to say "what financial position were you in when you had the accident/theft etc?" The answer to the insurers is how much would the bike have sold for if it had been sold at the date of the loss.
If you look at many adverts modified bikes don't always sell for a premium. Sure there might be a small mark up if there are substantial modifications.
If you declare your modifications insurers will usually put a standard parts replacement condition on your policy which restricts cover to that of a standard bike.
I understand the dilemma - I am really interested to find out if there is a sustainable formula for valuing modified bikes.
NBs996 - usually you are asked what mileage and an engineer will inspect so a reasonably accurate valuation can be given. Servicing records are also sometimes asked for and any other evidence to back up a value. Just how many write offs have you had?!!
