This is one of my favourite tunes (mainly because I like singing the bass line), and is fairly universal for 'It Came Upon the Midnight Clear' in UK.
To the best of my recollection, Sullivan arranged the tune from an 8-bar melody sent by a friend - the second half supposed to be his own.
It's also likened to the tune 'Eardisley', and there are similarities. Ralph Vaughan Williams edited the Oxford Book of Carols in 1928, and the tune of 'Dives and Lazarus' (No 57) there was noted in Eardisley, Herefordshire in 1905. It was also collected in Worcestershire much earlier.
I have references for all this somewhere, but it's some years ago, so I'll need to dig.
I've listened to all the tunes many times - I think Noel and Kingsfold are very closely related. I'm not so sure about Eardisley and Dives & Lazarus, but I hear some similarities. One caveat - I was looking for a different hymn tune on one occasion, and listened to about 30 of the same metrical pattern. By the end, I was hard pushed to tell any of them apart, so maybe I'm not the best judge.