He picked up his accent from his parents - his mother had a broad Scots accent until the day she died; also he was surrounded by Scots people throughout his childhood and early youth.
While he had a Salford speaking accent, according to his contemporaries the one he used for singing was that he got from his mother Betsy and other members of his family (the fact that he was singing her and other family members songs in the 1930s was confirmed by D G Bridson in his book on the BBC, 'Prospero and Ariel'). He neutralised his accent in order to sing the songs he had heard from childhood.
I wonder why nobody ever challenges the phony mid-Atlantic accent which is used by many singers, folk and pop, on the scene today.
Likewise, why does no-one ever query Robert Zimmerman's change of name?
Jim Carroll