Re Belle mouthing the words in the clip - I thought that it was probably something she would be almost unable to stop herself from doing!

I then read this interesting quote in Shelia's Obit in The Guardian, 17Dec2014 by Derek Schofield :

" ..... Her mother, Belle, was the source of many of Sheila’s songs, although until Belle’s death in 1997 Sheila was not always allowed to sing them. The family repertoire was large, however, and ballads such as Twa Brothers and The Mill o’ Tifty’s Annie were performed superbly by Sheila. It was Belle’s brother, Donald MacGregor, who taught Sheila, when she was still a girl, many of the family ballads learned from his father. It was not simply a matter of learning the words and tune. Donald instilled in Sheila the “conyach” – his word for the emotional expression in the song that comes from the heart not the head. He taught her all the ballads orally – Donald could not read or write..... "

Wish there were more clips of the younger Sheila singing!
In this clip, Sheila sings Mill o' Tifty's Annie through and Belle explains about "conyach" : https://www.youtube.com/watch?v=zZnorPhVjCo


R-J