Thanks everyone for this info. Pretty much what I suspected on the "tavers" verse, and as yet no convincing corroboration for SBG's "becalmed" verse.

Capt. B.: "why not make your own variation" is exactly what I'm doing. But I prefer to know before doing it whether the raw material I'm working with is actually that, i.e. raw material, or whether it has already been tampered with at some point. From the point of view of creating a song that hangs together and engages an audience this might not matter too much, but if I'm going to introduce it by telling that audience that I'm singing a "traditional song" then I'd like to know how traditional it really is, and how much of it may have been made up by The Rev. B-G or Bert Lloyd along the way. The more unadulterated collected versions I can examine, the more I can develop a sense for what is genuine and what may be a bit dodgy. Even so, there is always the possibility of a single singer coming out with a 'rogue' verse, like William Bone in the version of BGW he sang to George Gardiner: this ends with a verse in which young Johnny decides to write a letter to the victim's friends, to tell them she's drowned. I've never come across that line anywhere else.