I totally agree that Makem had no business copyrighting the song... period.... it was never his to copyright.
but it could be moot as to whether or not it had ever actually been copyrighted previously. It may well have roots in music hall but then been "folk processed" into a bothy song. That's where finding an earlier written version is important... which doesn't seem to be the case here.

A penny broadside ballad was rarely if ever signed, much less copyrighted... and the better ones entered the tradition/public domain, as did other composed pieces where we do know the composer. The bureaucratic insanity that I fear is where someone can make minor changes to a work like Amazing Grace and then claim a copyright for it.
Call me paranoid, but I knew in 1963 - as a sixth grader- that we were headed for big trouble with oil supplies if we didn't curb our wasteful ways... and was naive enough to think that it was obvious to any reasonable adult. Well, silly me. Now I know that short sighted greed trumps sanity ever time.