I just listened to a tape of "Wany of Man" and while Bok is the lead (and initially solo) singer, he'e definitely joined by Muir and then Tricket. It would be easy to remember the song as simply sung by him, though...   
Amd Joe, Dancing at Whitsun doesn't have the same melody as the much easier to remember I Once Loved a Lass. At least, I find the second tune easier to remember and I bet a lot of other people do too. That's the trouble! If you don't depart from the simpler melody on the third and fourth notes (I had to listen again to the tape to figure this out) you find yourself falling into the wrong song,   I don't know Birmingham Sunday (and am afraid to click on the link for fear of losing my iffy Mudcat connection), but if it's something in between, then no wonder people are confused!