Wonderful. Mick, you're a true star.

Since Caledonian Pocket Companion by James Oswald, 1747, appears to be the earliest written tune for the ballad, and all of these early sources are Scottish with Pepys making reference to the "little Scotch song" (notwithstanding what has been said above that this may refer to style - it may not), and that essentially the same tune was used in the 47 years from 1747 to 1794, from Oswald to Ritson, then it is at least possible that this tune or one very much like it was the same used for the 81 years from Pepys to Oswald's publication, especially when one considers the longevity of many of the tunes in John Playford's Dancing Master (Sellenger's Round, for example, was popular from its first known appearance in c.1595 until at least its inclusion in the last Dancing Master in 1728 – 133 years). All conjecture, of course, but entirely possible, I think.

Huge thanks.