Dear anonymous and scientifically-enquiring guest

Your version from the Walpole letter leaves out the last verse according to the Yale transcripts (p469 and p470 (and other copies of the correspondence). The Yale transcript also has a footnote:

  There came an old woman by,
  Their blessing she did crave;
  She cut her a branch of this true lover's knot,
  And buried 'em both in a grave[22]

[22] 'NB. Compare this song with Giles Collin, Fair Margaret and Sweet William, Lord Thomas and Fair Annet' (MS Note in Percy's hand). The ballad with many variations from HW's text, is printed as Lord Lovel in Child's Ballads ii, 207


The fact that they are so close doesn't preclude a common ancestor. Perhaps a look at Percy's papers at the Bodleian would shed some light?

Mick