Gee, I really don’t know. Maybe someone else will, but that was actually much better known as a poem than a carol before Britten. I know it’s not in the older Oxford Book of Carols, but if your library has the New Oxford Book of Carols it might have made it into that.
But if they want to add an accmpt to the Joubert version I suppose the best thing would be to figure out the chords from the unaccompanied vocal version, unless someone else here knows of a published arrangement.
The melody Sting uses is the same as in Rutter’s “Dancing Day.” The harp accompaniment in that particular movment is just a few chords with some vocal cues, so you’d be better off with the vocal score. Still quite sparse, but at least something.