"Folk music existed long before it was taken up by the left,"
Of course it did, but it lay pretty dormant for quite a while until the left started up the club scene and drew the attention of those outside the communities to it.
"....Henderson were unheard of."
You must be blood old - Henderson was one of he pioneers in Scotland, collecting songs and writing about them in the forties,
He was the main organisers of The Peoples' Concerts in Edinburgh, which introduced Jeannie Robertson, Flora McNeil, Jessie Murray, Jimmy McBeath.... and many other field singers to urban audiences.
"revival groups and singers were in no way left wing."
The instigators were - Lomax, both in the States and in Britain; he was recoding songs in the penitentiaries, with his father, and later he came to Britain and persuaded the singers here to stop pretending they were cowboys, and take a look at their own repertoire.
Organisations like C.N.D. played an immeasurable part in popularising our repertoires.
You'll be telling us Rabbie Burns voted Conservative next!
"were the first intrusion of politics into folk music, it gradually killed the genre "
They most certainly were not - MacColl and Lloyd were working on Industrial songs (in Bert's case, mining songs for the N.U.M.) in the forties) and MacColl was involved with the BBC, , with producers such as Bridson and Gilliam as early as the depression in the thirties.
What with Blair Peach, and now the left in folksong, you are what they call over here in Ireland 'a begrudger' - you certainly are the strangest 'socialist', I have ever encountered.
Far from ruining folk song, it's politics invigorated it - if anything killed the genre - it was the 'milk-and-water' approach that divorced the songs from their social roots.   
You really do need (desparately) to decide which side of the political divide you are on.
I suggest you follow up Muskies excellent point about the Chartists et al - a collection of the earliest known songs (in Latin and in Old English) goes back as far as King John (see Thomas Wright's 'Political Songs of England from the reign of John to that of Edward II'.)
In your own part of the world, the Jacobite movement was pushing inspiring songs as if they were going out of fashion and as for Ireland.... political songs form an enormous part of the traditional repertoire.
You appear to be as uninformed about folk songs as you are about left politics - if that were possible!
Jim Carroll