I was present at the folk club in Stratford, London, in 1971 when some of this was recorded. I saw the original broadcast, and my girlfriend of the time was visible but not me. We left the gig early because of the tone of the songs (Such as Ho ho ho Chi Min).
Unknown to me, Ewan McColl was well known as a communist supporter and was going through a phase of support and songs for the Vietnam war. Peggy Seeger is the sister of the late Pete Seeger and was also persecuted in the USA for her communist beliefs and exiled - which is the theme of the ducumentary.
I would love to find the program again.