If it was a folk song, I'd call the Darke a variant of the Holst tune. I note that it still doesn't get over the problem of including the words "Our God" in the second line, which means rushing the rest of the line uncomfortably.

I think Christina Rossetti would have been with me in thinking it better with the line shortened.(After all when she wrote it, the tune wouldn't have been in her mind.)

When it comes up in carol services (and I'm always pleased when it does), I tend to take a break at that line and come in with it as "Heaven cannot hold him - and I think most people in the congregation do the same. If there's a choir, let them worry about singing the impossible version.