Hi, Richie,
Thanks for posting this text too. Where does it come from? I've just been looking in Campbell and Sharp's "English Folk Songs from the Southern Appalachians", which has Jeff Stockton's versions and four others, going up to "Version E" – well short of "Q".
   
Well, as far as chicken and egg goes, I guess the ballad had to be there before the broadside - you can't print a song until there's a song to print. The question is just how long and how much happened to it before it was printed. Was it written for the press, maybe by someone who had been having a few drinks with Charles Stewart, and printed straight away, or was it already circulating in some form before the broadside printers got hold of it? And if it was already circulating, had it started from an incident involving Gosport, the Bedford, and Charles Stewart in 1726, or were these specific names added to an existing song to make it sound more convincing?

Short of a definitely pre-1726 manuscript or printed version turning up, I don't see that there can ever be a definite answer to these questions.If there was something common to a lot of traditional versions on both sides of the Atlantic but not found in broadsides, like that stanza you pointed out in "Lord Thomas", then that might point to a tradition independent of print, but is there anything like that?

In the meantime, a broadside text inspired by the events on the Bedford as unearthed by David Fowler looks a likely enough starting point to me (though probably with a junior carpenter as the murderer rather than poor John Billson, as Paul Slade argues at the end of the article you mentioned earlier). But what I find most interesting is what broadside writers and singers have done with it since then – paring the original 34 stanzas down to a few essential and powerful lines in "Pretty Polly", adding the avenging ghost that tears the murderer in three in the English versions…

By the way, have you come across this recording of the long ballad performed by a traditional singer in Orkney in the '60s? http://www.tobarandualchais.co.uk/en/fullrecord/63592/1