This was written for Christian Aid week in 1965, and Sydney Carter (no relation, despite my attempts to discover otherwise) described it as a song, not a hymn. According to Hymnary.org, it is sung as a hymn to no other tune, and the tune is used for no other hymn. It is very simple, unlike, for instance, The Crow on the Cradle (which may be something of a collage), or Lord of the Dance which is an adaptation of Elder Joseph Brockett's tune for Simple Gifts. More like One More Step. Sydney was a poet primarily, I think the tunes he did write were mostly pretty simple. These two songs, together with Lord of the Dance, and amongst the most used in Primary School assemblies in the UK.