Sung mostly by the Bird Woman, although Mary Poppins does sing a bit of it. She's not sitting on the steps of St. Paul's as she does in the film though. I thought it was lovely, and, as that song is one of my favorites from the film, I got a bit emotional as it was sung... *blush*
Funny thing... on one of my trips to London a few years ago, I was in the area of St. Paul's on a Sunday morning, on my way into the cathedral for the service. I found myself looking right at the spot where Jane Darwell sat with her wares, absently thinking about the song!