Brian,
There are numerous examples of what you are after.
Fred Jordan sang plenty of songs he got from people in the revival.
Roger mentioned above only started singing after Frank had died, at the instigation of Ian Russell who recorded his dad.
Bob Copper was a collector and writer.
All of the traditional singers who now sing at the festivals have learnt material from sources out of their own tradition. Will Noble sings songs from the Haxey Hood game. He picked up some of his songs from the radio.
The Scottish travellers almost all augmented their repertoires from books.
By the way, Matthew, it was Bell Robertson, not Bell Duncan. Duncan was Greig's fellow collector. An understandable slip.

An interesting example is the Whitby version of 'The Tailor's Britches' from the singing of Arthur Woods, currently being sung by John Greaves and Martin Carthy. An important bearer of the tradition, Arthur probably rewrote this excellent version himself. the only other versions known are 4 fragments from the south coast, not published until after Arthur had been recorded.