Tried to post a message, Fred, but it disappeared in the ether!

I also looked in Greig - Duncan but found nothing, and I'm not altogether certain that SOTS is a variant of FH. It certainly shares a lot of the common 'floating' motifs but IMHO it moves away from the abandoned lover theme and takes up a more egalitarian notion ( the contrast between drinking wine or tea/ the explicit statement of social mismatch between the son of a king and the daughter of a poor working man).
It just seems to me like a song that has been put together by a competent maker who wanted to express her/his own opinion of the problems of inequality, and did it through the vehicle of a love song….

Anyway, Fred, as I've said elsewhere -- the song came to Ian Davison c. 1964/5 when he was in charge of Rutherglen Academy Ballads Club following the election of the late Norman Buchan to Parliament. Ian got it from a pupil, Kathleen Mitchell, who 'had learned it from her grannie'. He gave it to me (we were in a group at the time) and it was also sung in the Club, of which Mary Stewart was a member -- but she never collected it directly or learned it direct from oral tradition.