If Bert's source was Buchan then it is just as likely Buchan was up to exactly what Bert was doing. Most of Buchan's ballads in Child for instance are under great suspicion. Child himself held Buchan's output in great contempt. It is widely known he added much material to his versions and even completely wrote some of them, claiming (like many of his predecessors and contemporaries) to have collected them from the mouths of ancient peasants.

I have a copy of the SSS mss from Harvard and will compare it with the above broadside when I have a little more time. Buchan got his 'inspiration' from many sources, one of which was English broadsides. He was for a time a broadside printer himself.