Oh, hurray, thank you, Rosie! FindMyPast has evidently lost them somewhere along the line as I looked for Maria for quite a while this morning. I've just dug up the announcement, in 1923, of an impending marriage for Ian (of 13 Dundas Street, which I think tallies with that Electoral roll entry you found earlier) and a Peggy Thomson of Valleyfield, Helensburgh. The penny has also just dropped that the "G.M. MacKenzie, niece," who was the informant on William's sister Margaret's death record, must be Gladys Maria, so this is turning out to be a very good day indeed!!!