I saw Ewan McColl and Peggy Seeger in concert many years ago, and he told the story of how it came to be written.

Peggy was working on a TV special. He called her at the studio, and when she came to the phone, she said, "This may sound funny, but I need a love song that's a minute and three quarters long."

He said, "All right, there's a melody that's been going through my head." He hummed it to her, then wrote the words to go with it while on the phone. When sung at the tempo it was originally written for, it took exactly a minute and three quarters to sing.