John Macreadie told me the story recently of his song, "Doomsday in the Afternoon" being used in a play in Edinburgh about Travelling People where it was credited as Trad. When it was pointed out to them he at least got some free tickets to the play...
And.. I always thought (inversely) that Michelle Shocked had written "The L&N don't stop here" till I came across a version of it that credited it to "Trad, arranged J. Ritchie" and later still to another that said she'd written it. Now I don't know what to believe...

Andy