I'm the author of the blog post quoted on 10 August 2010. Gentle Angry People is long out of print and has a number of radical hymns. The chorus of Sing John Ball is as above. There are 3 verses in GAP:

Labour and spin for fellowship, I say
Labour and spin for the love of one another
Labour and spin for fellowship, I say
In the light that is coming in the morning.

Green are the leaves and merry is the May,
Long is the night and the winter that is over
Green are the leaves and merry is the May,
In the light that is coming in the morning.

All shall be ruled by fellowship at last,
All shall be ruled by the love of one another,
All shall be ruled by fellowship at last,
In the light that is coming in the morning.