The YouTube recording Jack Blandiver linked to, had this message below it:

The melody is a traditional carol, found and arranged with these words (thought to be of Shaker origin) for CHOIR by English Composer, Elizabeth Posten. It is not Jane Sibbery's song, nor Jane's idea. Her version is beautiful, however, listen to one of the British choirs sing it - it's beautiful any way it's done.

I added the title and to Jack's post. It is indeed a beautiful song. There's a thread on the song here (click).

-Joe-