Singers like Peter Bellamy and Louis Killen were long-time friends and followers of the Copper Family (see "Related Videos" in the YouTube page for an example} who have a family tradition of harmony singing going back many generations. The harmonies sound somewhat strange to people used to classical or standard church harmonies, but are related to arrangements used in West Gallery music. Familiarity with a wide range of such material is just the starting-point.

Some people have the ability to harmonise at will, and I would say Louis Killen would not have had a great problem finding and setting a harmony line after a couple of runs through the song (and I don't mean in any way to diminish the skill involved). If they were modelling on an existing arrangement (this could be a Copper Family song, I'm not sure), so much the easier.

I can do this myself, and have been lucky enough in life to find a few other people capable of working like this. When it does work, there's nothing better.

Ross