The story of Sydney Carter's association with Donald Swann is detailed in Carter's obituary in The Guardian:

https://www.theguardian.com/news/2004/mar/17/guardianobituaries.religion

The entire obituary is fascinating, but here's an excerpt:

With the second world war, his critical spirit and abhorrence of violence led him into the Friends Ambulance Unit, with which he served in the Middle East, and, in 1944, in Greece, along with a stim ulating group of pacifists, including Donald Swann (obituary, March 25 1994). If any church could come to holding Sydney's allegiance, it was the Society of Friends, with its rejection of dogma, and its reliance on personal experience and social activ-ism, and its affirmation of God's presence in every human being.

After the war, folk music, both sacred and secular, took Sydney over. Much influenced by what he had heard in Greece, he studied its many forms; then, in 1952, he started writing lyrics for Swann, who needed revue material. "I found out that I could do that," Sydney said, "and get paid for it."

He launched what proved to be a long collaboration by providing lyrics for Swann's composition The Youth Of The Heart, which featured in the Globe Revue in London's West End. In the mid-1950s, he was the lyricist on Swann's children's musical, Lucy And The Hunter.

I was a BBC producer when, in 1960, Sydney wrote his most controversial song, Friday Morning. I believe it was also one of the most profound. In it, the robber, crucified with Jesus, cries out:

It was on a Friday morning that they took me from my cell
And I saw they had a carpenter to crucify as well.
You can blame it on to Pilate, you can blame it on the Jews,
You can blame it on the Devil, it's God I accuse.
It's God they ought to crucify, instead of you and me,
I said to the carpenter a-hanging on the tree.


Classic theology says that it was God, but Sydney lets the irony stand. In this, as in the following stanzas, he piles on the guilt, piles it on to God. It leads to the deepest of all questions: is God in Auschwitz or the Twin Towers, the killer or the victim? If there is a God? I had to fight the BBC management to get that song on the air. A brave, liberal head of religious broadcasting was my ally. Today, the fear of a backlash would be far greater.

In 1962, Carter teamed up with Sheila Hancock for the album Putting Out The Dustbin, one track of which, Last Cigarette, on failing to give up smoking, became a minor hit. The songs on the LP were closer to cabaret than to folk, but the pacifist, political singer was there even then. In 1964, the Donald Swann EP, Songs Of Faith And Doubt, comprised six songs by Carter. In the 1960s too, he worked as a critic for Gramophone magazine.