What is the main source of this tune that starts high and descends? I only know it from the singing of Ian Manuel but I think it comes ultimately from the Scottish travellers, Jeannie Robertson perhaps.

I first encountered it when singing round the piano with some friends of the family in New Zealand in the mid-60s - it was in a book they had, and I can't remember what. So it wasn't Jeannie Robertson who made it known.

As you say, it's not what most ballad tunes do. It's still one of the standard patterns in Near Eastern music, but seems to have dropped out of currency further west.