This is unbelievable! I recently asked my insurer (Pantaenius) to quote me for Sea-hawk "Puffin", and without including the date of manufacture (as I didn't know), they came back with the full dimensions of the boat AND it's year of build! 1978.
I have checked the data on the forum and by interpolation their date seems to fit the assumptions made in the 'Buyers/Dating' section. How strange is that? Maybe they have a register of boats manufactured in Uk or something?
I don't know Pantaenius as a company, but perhaps they are part of a bigger group and that one of the partner companies have insured her in the past? If there's been no name change and the sail number ties up, then it's highly likely that it would be the same boat.
Funny, I was thinking the same thing as Greg, but he beat me to it to write it down. I doubt there'll be a National register available for these little boats, but most companies use 'guideline tables' to be able to have an estimated value of the item.