While I have the utmost respect for Kenneth McKellar, I'm unable to enjoy his version of this song. It's too much of a ringing sound he gives it.
I feel it should be light and fanciful, which it is most of the times. Sung in 4 part harmony is fine if 3 of the four parts just do the "Humm" version of the song.
The song is best done in a simple form, in my opinion.