Since writing the last bit.. I do find on the PRS/MCPS database that their registration credits McGinn with the words, but they have rightly or wrongly allocated the tune as trad. and many releases on their database state that.   So if that is correct, McKellar was legally entitled to come up with his own version using the same tune and changing the title and theme.   There are actually two registrations, one with full song credited to McGinn, the other as above.