The same-tuned song "The Burning Babe" (poem by Robert Southwell, setting by Mike Hockenhull), which I have in Free Reed's "Midwinter" CD anthology, was credited as "tune adapted from Maddy Prior's 'Somewhere Down the Road'" (both items of misinformation very much sic). The liner notes clearly needed a good copyedit, though the compilation—all 4 CDs of it—is actually quite good.
So no light shed there on the tune's origin, Joe. But I think its resemblance to "Nearer My God to Thee," though admittedly striking in the first few bars, is pure happenstance. It doesn't hold up as the song continues.