It's not my carol -it's "Angels from the realms of glory", no 119 in the Oxford Book of British Carols, first printed by Montgomery in his newspaper "Iris", 24 December 1816 (that's the carol, not the Oxford Book which was published over a century later). I think Frank Purslow would have known about this! RVW and Martin Shaw arranged the tunes for the Oxford Book of Carols.

Words, J Montgomery
Music, Old French tune
Arr., Reginald Jacques
Oxford University Press