The song was originally included on Transatlantic TRA 126 - Second Wave (1965). It is credited to Causley/Pegg, the former of course being the poet Jeff mentioned above and the latter being Bob and/or Carole Pegg. Discogs also has an "Atterly" attribution slipped in there, but I'm not sure where it's from -- the disc credits and liner notes don't attribute it to him.

The album's liner notes (link) say that the song is the poem by Causley set to music by the Peggs. You can read the poem here, in Poems Of The Mid-Century by John Holloway.

You can hear the song here. The lyrics match exactly those of the poem, including the verses of Envoi.