A little more on "Hark." Words were first changed by George Whitefield in 1753 (didn't like word "welkin" and changed some lines). Whitefield set the hymn to music that was later discarded- this probably is where the "Cookson" came in, haven't been able to find it.
In 1855, Cummings, organist at Waltham Abbey, set the hymn to music ("with changes") from the 2nd chorus of Mendelssohn's Festgesang No. 7, commemorating Gutenberg, the printer. Cummings also changed "King of Kings" to "newborn king" (and other changes?). Mendelssohn expressly asked that the music not be used for sacred texts, but he was dead by the time that Cummings borrowed it. The tune should be properly cited as Mendelssohn arr. Wm. Cummings.
The date 1885 which I mentioned in the previous post may be a misprint for 1855.
Publication apparently was in Chope, 1857.

A more famous William Cummings was the first curve ball pitcher.