Jack Campin wrote -
" Many of the very oldest songs we know seem to be lyrical fragments extracted from much longer narratives"
Undoubtedly true! I would give Child 19 King Orfeo as a prime example. The story of Orpheus and Euridice from Greek mythology turning up in manuscript forms in the 19th century in Shetland being the ones Child includes and fragments of that ballad being collected in the oral tradition there in the 1950s.

Jack Campin also wrote -
"I haven't read Roud's book yet...."
Then I would suggest that someone like himself really ought to do so quite soon.

Can I also thank Jim Carroll for his enormously thought provoking quotation from The Ballad Tree. There's another book that I will have to get down from my bookshelves to give renewed consideration.