Leeneia - my At Least Four Assumptions are entirely consistent with the OP. What makes you think otherwise?

1) If they didn't have a recording - how would they know the song?

2) If they didn't play an instrument, why would they want the sheet music?

3) If they were skilled enough to sight read, then experience teaches me that playing by ear can come easy with but little practise. It's the Folk Way.

4) If additional musicians are involved then - the more the merrier really, although it might be odd to have more than one fiddler playing off the sheet music to so simple a wee thing as Little Musgrave, but nothing would surprise me in the folk world.

Whatever the 'right' tune might be, the one they want is the Plantxy version, which derives, I believe, from Nic Jones.

And if I wanted be called Sublime Astray, then that's the name I'd have chosen. A little respect goes a long way - even by Mudcat standards.