Hi there Galium, and many thanks for your reply.
Winifred was indeed a Wiltshire gal, as confirmed by other members of the family. You're correct re: Maxwell's marriage to my grandmother, Violet, and my father remembers her living until a ripe old age, so I definitely think 1975 is a good fit on that score, and she was indeed Winifred Ellis living in the Devizes area by then...obviously more detective work is needed here to be 100% sure, not least because of the common surname, and if the marriage cert proves unsatisfactory I can always order the birth certificate of her second son, Harry, which presumably would be more exact about date of birth and birthplace...