"Bob Davenport is notorious for his bad behaviour towards audiences and performers alike but I would think the animosity between them had more to do with Bob's insistence that 'Urban Spaceman' and 'September Song' were folksongs."
I must say that I have never experienced any bad behaviour from Bob,on the few occasions we have met, I have always found him polite, and I dont think that is because he is smaller than me or that he has heard of my pugillistic skills.
I think this statement about the cause of the animosity does not tie up accurately with other things you have said, you have stated on this forum that there was a fight between them at a meeting.
This meeting was held to discuss the direction of the UK folk revival, amongst those present were various people who had met to discuss not "September Song" BUT wanted[ as I understood] to DISCUSS the direction of the UK Folk revival these included MacColl and Davenport, to suggest their animosity was about Bob maintaining the The Urban Spaceman was a folk song I find hard to believe, it sounds more deep seated than that.
The fact of the matter is that a number of people did not like him.
I decided never to visit the Singers club as a result of what I considered was his boorish behaviour,on the other hand, On a later occasion I heard stories of his kindness, a relative of mine was allowed to use his archive material and allowed to go into their empty house, Ewan and Peggy had taken considerable time in locating the appropriate items for their study.It was a notable example of kindness and helpfulness
It seems to me    that MacColl [as a result of Alan Lomax suggestions[ your own words]along with others notably Carthy, Killen, Cyril Tawney encouraged people to seek out songs from the Geographical BRITISH isles if that was their background,they managed to influence the direction of the UK Folk Revival, regardless of any meetings or fisticuffs.
The Singers club rules discouraged the singing of American songs [unless you were American] in their club, out of curiosity, What was their attitude to Woody Guthrie songs being sung by English singers?.