If indeed Scarborough Fair is a variant of The Elfin Knight, it should be
in Child with a zillion other variants.  It doesn't remind me much of the
variants I do know, but that's not uncommon.  Pretty Polly is said to be
a variant of the Gosport Tragedy, for example.

Of course, Child doesn't have any tunes.  I too have heard a number
of recordings of Scarborough Fair, but only S & G, and one post-S & G
recording use that tune.  That doesn't prove anything, of course, and I may
well be mistaken in my assumption that the tune originated with S & G.

Richard Todd