Huge IMO. I wouldn't buy a used 7 without one and would always advise anyone buying one to ask to look at this first. Doesn't matter what plate it has on it.
The sort of owner you want to buy from keeps all the papers to do with work etc he/she's had done on it.
Problem is, if you're not about to sell it's more an inconvenience than something of actual worth.
I had a dealing with a removals firm recently who "lost" my belongings. Said they'd been shipped abroad and had been disposed of by the recipient etc. Told them to send people over to check, the whole 9yds, no dice.
We put in a massively detailed claim that amounted to a not inconsiderable sum and lo, the "lost" items were all found within a few days.
Come up with something plausible - say the difference between historied and non-historied cars, adding credence to your info by getting an idea from Caterham and other sales places - and present it to them (could easily be 500-1k I would guess). I reckon that they'll find it given adequate prompting to tunr their place upside down.