I love it when people state unequivocally who a well-known song was written by or about, and it's a relative of theirs.
I know of three totally unrelated people for whom John Martyn personally wrote the song May You Never. In each case it was a relative of the person who told me about it.
Mind you, I'm sure Luke Gaulton is absolutely right. I just wonder what evidence he has, other than the statement of a professor - I've known professors who profess to know many things.