PS - I think it's almost certain that Chappell didn't get the tune from Motherwell, because Chappell wrote:

"A copy of the ballad is in the Bagford Collection, entitled "A lamentable ballad of Little Musgrove and the Lady Barnet, to an excellent new tune." It is also in Wit restored, 1658; in Dryden's Miscellany Poems, iii. 312 (1716); and in Pcrcy's Reliques, series 3, book i.

I can't imagine why he would cite all those sources and yet omit mentioning Motherwell if he had really got the tune from Motherwell.