Yes, I had mentally placed C# as 18th century, but true, he started in 1903. So I agree a case can be made for him, as it can for Lloyd, Coppers, and Hugill.

But as the (I think that is not too much) pivotal figure of English folk music for 40 years - still, I think Martin Carthy. Ask it the other way round. Remove him from the soup and scene from the 60s onwards, and just so much would have died or never been born. He has a connection to the research and historical side of folk music that few performers, and none of his calibre, can equal. Or put it another way, ask the public to imitate a folk singer and they immediately put finger in ear and sing through nose in the style in which he was so often seen in his early years - in short, they impersonate him. He is the archetype, the scholar, and the pinnacle.

MacColl of course was an influence, but I can't get past his falsity in pretending to be a different person, and of course despite his traditional work he is now really noted for his contribution as a singer-songwriter - not a folkie.

The other possible mention here might be the Young Tradition (they have to be considered together) who defined English vocal harmony in the 60s and whose sound still defines it to this day.