Text, midi, Noteworthy and Sandys sheet music, along with an explanation, is given in the Hymns and Carols of Christmas website: Tomorrow

I am surprised that there hasn't been more discussion of this carol. Guest Rae, thanks for bringing it up.

It was posted by 'T in Oklahoma' in thread 7425 in 1998: Christian Folk Music
Maddie Prior's version is in the DT.