Martin, your connection to the WMA is spot on!

I should have looked here before, but the British Library Catalogue has one, and only one, entry for 'Edward Brangwyn'; he is noted as the composer and author of the song People's front - unison song with piano accompaniment (staff and tonic solfa), published by the Worker's Music Association c.1937.

This must be the same song as that which appears in the Canciones de las Brigadas.

Matthew