I did say nice examples will always be sought after. Your Silver arrows was better than most hence you got a higher price. Nick's last car had around 25000 miles which probably makes it the lowest mileage SL60 in the country.
An average one, with average miles won't make much more than book.
Actually my point was regarding certain models - not mileage or condition related. I've yet to see any Silver Arrow sell for less than £22k - regardless of what the 'book' says. Most go for £25k at dealers.