I’m pretty sure I first heard this song in a pub in Adelaide, where my university choir used to rejoice in making up silly lyrics for the verses. Any rhyming couplet with twelve syllables per line would do – I was always fond of ‘Mary had a little lamb, the doctor was surprised! When Old MacDonald had a farm, he couldn’t believe his eyes!’.
Most of the other verses were ruder than that, naturally.
But in its original form, it’s rather a lovely carol. I’ve sung it with a number of sensible carolling groups over the years, but the only one which ever turned the tenor line into the descant was that old pub choir. I think they had the right idea. But then, I never did see a descant I didn’t like…