As I say, the singer of Brit Elkland's naked dance (AKA Willow's Song) is Shiela Mackie, who also sang the masterful Highland Widow's Lament that opens the usual / shorter cut of the film, and many (myself included!) have mistaken for Ray Fisher.

And whilst we're on: http://www.youtube.com/watch?v=HdWY-AMY_zY