Dave, I can well believe that they wouldn't have sung work-songs in harmony, any more than soldiers would harmonise on a jody. But they may well have harmonised in the pubs - plenty of what we call "shanties" because of their nautical subject are actually drinking songs that wouldn't get sung at sea.

And I'd be very surprised if there was no tradition of harmonising in Britain. For centuries, the church was the high point of musical theory, and the religious songs of the mediaeval period definitely have harmony elements. Musical notation was pioneered by church singers as a way of recording how the melody and harmony lines were supposed to go. So it'd be quite surprising if other "home-made" music didn't include harmonies.

Graham.