Have you checked to see if Herbert was killed during the war? That might explain Euphemia running off to Australia ...
If the Nottinghamshire Herbert, who was single, is the right Herbert, I wonder why he and Euphemia didn't marry. Isn't it the case that if a couple is unmarried and registering a birth then the father has to be present? If Herbert is named as Marion's father he must have acknowledged the fact and you would think he would have done the "decent thing"

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Carol, when you wake up, can you check to see if there is any record of Herbert going to Australia?
I also would be interested to find out why after such a long time away, that an elderly Euphemia came back to Scotland in 1959. Did she die in Scotland?