MtheGM:
I know the Glasgow Orpheus Choir sing it (in Gaelic), and he does the famous recitation, but doesn't he sing it (in English) as he marches to the castle to meet his fate, near the end of the film?

Incidentally, Livesey never went anywhere near Scotland during the filming - he stayed in London the whole time - they used a double for location shots of him in the Scottish landscape.