Even if you were able to turn up a tune from around Pepys' time, you would likely be disappointed in its "excellence". If you treat the "Flanders Shore" tune freely, as indeed you must for the current text, it's really not difficult to retrofit it to "The Unnatural Mother".

Hammond recorded that Mrs. Notley said, "The father locked her in a room so high"; this line occurs verbatim in "The Ploughman's Love." While Notley's version shows signs of previous folk revision and condensation (such as the relocation of the meeting to Flanders), her comment suggests to me that she had once known (but forgot) more of the song, and the confused chronology probably resulted from her singing the verses in whatever order they occurred to her. A memory lapse would also explain why the last verse is partial and misordered.

To me, this song demonstrates the misplaced reverence we place in traditional singers as sources and in the "wisdom" of the folk process. Modern singers continue to parrot this overcondensed mishmash simply because Notley happened to sing it to Hammond this way, and Jones (likely lacking access to other versions at the time) followed suit.

Being a bit more rational, I reconstructed a six-verse version of "Flanders Shore" which straightens out the chronology and provides a quick forestory modeled after "The Ploughman's Love". It does not attempt to explain the sense in firing the bullet nor the cause of the light springing from her clothes (given her state of decay by this time, I'm guessing spontaneous combustion). Even "The Ploughman's Love" doesn't adequately explain these points, and adds the ghoulish touch of the young man firing the bullet to where she lay while she was still alive! Of course, given the armaments of the time, it would have been a supernatural bullet indeed which could traverse the channel, much less hit a seagull accurately at fifty paces.