As Dave suggested is the only way to get them to pay you what you think it's worth.
When i wrote my limited edition Mini off a few years back now, they first offered me £1100....I paid £1650 for it & knew it was still worth that. All i did was photo copy magazines etc to show its worth & eventually they paid me £1650.
Then again they had to really, as i had already been notified by the DVLA that somebody had tried to re-register it...so they'd obviously already sold it!
Good luck mate....you going to get another NSR?
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88 NSR250 R2J (work in progress!) & 05 Aprilia SL1000 Falco