The following is just my opinion, for what it's worth, after several days of perusing the Malcolm MacFarlane melody and what has been arranged around it, of "A Lament for Nurse Cavell."

I get an uncomfortable feeling when I hear the sound of this composition all together:
Arthur W. Marchant's churchy chord progressions and voice-leading in the harmonies;
Malcolm MacFarlane's modal, traditional-sounding melody;
and Murdoch Maclean's nineteenth-century-sounding English verse.

This feelings persuades me that the whole composition was sort of cobbled together, that it is not of a whole. Sadly, it sounds like less than the sum of its parts.

On this forum thread, the initial post and the eleventh post both give author/composition credits as printed in the sheet music for "A Lament for Nurse Cavell." What is quoted in the original post, comes from the frontispiece of the sheet music, which has then been duplicated for the archives catalog records. What is quoted in Post Eleven, dated 17 November 2016, is what I found printed inside the score, directly above the opening measures of the song.

"Melody and Gaelic words by Malcolm MacFarlane" is more specific than what is printed on the title page, "Music by Malcolm MacFarlane....Gaelic and English Words."   

The following is my speculation only.

Malcolm MacFarlane's tune, and his Gaelic lyrics, are of a piece, and fit together hand in glove, the most natural thing about this commemoration song. Murdoch Maclean's English poem can be made to fit to Malcolm MacFarlane's melodic line, but the fit is not without awkward and unnatural phrasing. I believe that poet Maclean wrote the poetry on its own, having no connection whatever with the tune. I will never know whether MacFarlane was required to take Maclean's already-finished poem, and by himself come up with a Gaelic version and a melody to complement Maclean's English lyric. But that is what I speculate. I go on to guess that, faced with this challenge, MacFarlane came up with a Gaelic text which, while not a literal translation from Maclean's English, ended up being a free adaptation of Maclean's thoughts and sentiment into a Gaelic poem which -- I presume to say -- stands on its own merits. Certainly the melody and the Gaelic prosody fit each other better than MacFarlane's melody fits Maclean's English verses.

Arthur W. Marchant tried for a somber, tasteful effect in harmonizing MacFarlane's melody from the top down. But Marchant is working from a classical-music and High-Church context, while MacFarlane's melody is neither of those things. Marchant's chords and counterpoint attempt to persuade the ear that the whole piece belongs in the diatonic minor that is conventional in church hymns. MacFarlane's tune, however, is modal, specifically mixolydian, and it rises and falls in a way that church anthems rarely do, composed as it is to sound like oral traditional singing.

So I guess this whole package, this way of selling the product as it were, was an attempt to be all things to all people, appealing to the English on one hand and the Scottish/Gaelic/Celts on the other. Nice try, but I'm not surprised that nobody has heard of the thing.

That front-page blurb references "celebrated Gaelic tenor," a Sergeant-Major Colin MacLeod. I imagine him singing MacFarlane's Gaelic words to MacFarlane's melody, rather than singing Maclean's English lyric. And that is probably the interpretation that would find more favor and have more success than the other approach.