Good to see some interest provoked! The "extra verse" only appears in Jerry Silverman's The Panic Is On, who I believe wrongly attributes the song to Bill Wolff; I spoke to one of Bill's friends in California many years ago who confirmed that. Other clues include the Unity Theatre, associated with the Workers Music Association. I also wrote Barrie Roberts, Alfie Bass and Aubrey Bowman with no success. Five years ago when I was in London the WMA was still active but I was unable to make any direct contacts.Anne Schuman was their Honary Secretary, 240 Perry Rise, Forest Hill, London, SE23 2QT, Tel:0181-699-2250. They were still doing sings at Covent Garden Community Centre in London.
Someone want to follow this up? In some ways I'd hate to see the mystery resolved – the search has been so much fun!