Tracy and I sang this often in the car while the boys were growing up.
I learned today that a "calling-on song" comes from rapper sword-dances, more commonly found in the north of England than the south. Rappers are not actually swords, but two-handled blades used for cleaning the pit ponies that were used in the coal mines. The blades look rather like the drawknives one might use to make baskets from ashwood here in NH. The dances are much like the group sword dances of Scotland, with intricate and vigorous steps. The calling-on song is the introduction, in which the 4-7 rapper characters are introduced by being called out one at a time. The traditional characters were similar to those in mummer's plays, but gradually a set group of men purporting to be the sons of famous military men - Nelson, Napoleon - came to dominate.
Ashley Hutchings, then of Fairport Convention, reworked the Winlaton version, eventually recorded in abbreviated form by Steeleye Span, as above.
Old folkies, especially old English folkies, sing in harmony and interact so pleasantly with their audiences that one automatically assumes their relations are generally, er, harmonious. This is completely untrue. The lineup which sang this version in March had already broken up by April, some of them vowing never to perform together again. And they never did. This tends to be true of performers in general, but perhaps more so among those who are trying to do something original. One wants to be more Irish, another wants to be more Renaissance, a third pushes for more electronics, and the fourth and fifth are going through a divorce.
The wonders of the internet. We can all free ride on the enormous work that some fanatic (or unlucky graduate student) did to preserve knowledge. You can learn far more than you ever thought was available about calling on songs (also called pace-egg or peace-egg songs), rappers, and the remaining sources of knowledge about them at Free Library. Well more than even I would read. And Part II references every town in the NW of England that has ever been connected to calling on, rapper, souling, pace-egg, or hero-combat play. About 400 in all. You will note that many of the towns listed no longer exist.