John Moulden, I covered all these tunes and lyrics in my dissertation which looked for the comparison between Tannahill's chosen tune 'The Three Carl's o' Buchannan' and the tune set to it about 10 years later by R A Smith The Braes O' Balquhither '
I got a copy of Tannahill's letter to in his own hand to Smith with all the lyrics including a verse not usually sung.
I also found a music sheet of the Three Carles tune printed in 1810 and pointed out the hybrid version of the Braes and McWilliams 'Lass among the Heather' sung by Jeannie Robertson. Greig-Duncan didn't seem to know about McWilliams which was surprising and Hamish Hemderson said that John MacDonald sang the Three Carles where in fact the tune was McPeake's Wild Mountain Thyme! That was a mild shock.
John, your booklet, The songs of Hugh McWilliams, was a great help and I managed to compare the tune Saint Helena to the Three Carls by oral transmission.
.... and much more, the research was great fun.