PEDANT ALERT
They claim the carol hasn't been performed in over 500 years but how do they know? That may well be the age of the manuscript, or of the book containing it, but what's to say the carol wasn't still being sung after that date, from other written copies? At a guess it would have dropped out of use during the Reformation, but even so, might have been revived during Mary Tudor's reign. Which makes it 450 years rather than 550.