The line in question was, in fact, borrowed from Hamish Henderson's 'Elegies for the Dead of Cyrenaika' (first elegy: 'End of a Campaign'), as Brian McNeill acknowledges in his songbook 'The Back o' the North Wind'. He also says (frequently!) that the song as a whole is a product of his desperation with the state of Scotland today (well, yesterday - before they had their own parliament, but I'm not entirely sure his opinion has changed a lot since!)