Thanks, Bruce, but unfortunately the history related in the other thread is more of the manuscript-tradition type. It doesn't address what the song, if it is more than a straightforward narrative, might be about. One reason to take it as an allegory is the rose that grows from Sir John Grey's grave. His is an explicitly English, as oppposed to Scottish, name, and the rose often stands for England, as in the House of the Rose. Witness "the red rose of England shall flourish no more" (The Death of Queen Jane).

P.S. Is there any way to combine the two threads so that you don't have to call them up separately? It would save time both for those with the questions and those with the answers.