>A serious difficulty is drawing valid conclusions from quite limited evidence.

Absolutely. And indeed there's not much evidence in the south-west either – Motherwell's manuscript just has a single text (compared with multiple versions of "Jamie Douglas", "Hind Horn", and "Mary Hamilton", and two of "Tam Lin", to name but a few), and the Crawfurd Collection only has one mention in a list of song titles (according to Emily Lyle, who edited the collection, probably a list of songs known to Mary McQueen, the greatest contributor of songs to the collection, a servant in Lochwinnoch, Renfrewshire, not far from Kilbirnie, from a traveller family, who later married a weaver, and emigrated to Canada in 1828). It doesn't add up to much evidence of popularity. As for Sharpe, he may have heard it sung in Annandale with the ships and similar stanzas, but he doesn't say anything about how much it was sung there (if indeed he had any way of knowing) -- and he doesn't include it in "A Ballad Book".

By the way, I'm in no position to take sides in the Peter Buchan debate, and gladly bow to Steve's knowledge of Buchan's working practice, but I just wonder is it possible that the stanzas in Buchan's text with Captain Green, Peg Morton, etc., rather than being his invention, come from some sort of ironic local parody that he had got hold of and clumsily pasted together with bits of one or more traditional versions he had collected?