With reference to Gordon Bok's "Play of the Lady Odivere," Jack Campin asked, Isn't that one King Orfeo rather than the Great Silkie?

It's neither, but it's a lot closer to the Great Silkie. Same broad plot, but the Great Silkie doesn't have a Sir Odivere.

"King Orfeo," cut down from the romance of "Sir Orfeo," is the Orpheus legend with a happy ending. Euridice, called "Heurodis" in the romance and "Isabel" in surviving versions of the ballad (all of which are very fragmentary), is captured by the King of Faërie with his rout. Orfeo abandons his kingdom to his steward, taking only his harp, and sets out to find Heurodis/Isabel. He sees the Faërie hunt and follows them; he plays so well that the King of Faërie makes a rash promise to reward him. Orfeo claims Heurodis/Isabel and returns to his kingdom, which the steward has faithfully ruled.

Since "Sir Orfeo" is in the Auchinleck Manuscript of c. 1330, and it can be shown that "King Orfeo" derives from "Sir Orfeo" (this was the point of my book Romancing the Ballad), "King Orfeo" is, in its roots, one of the oldest of all ballads. But it has no silkies. :-)