Going by birth details 1911 has this family at Llandegai all born Bethesda. Ellen is age 8 down as Nellie a pet name for Ellen. William is a Water Balance Brakeman.If access, 1911 - https://www.ancestry.co.uk/search/collections/2353/records/9197251921 same family/area also down as Nellie age 19 yr 1 mth which would fit with a May birthdate as census was in June.1921 - https://www.ancestry.co.uk/search/collections/63151/records/35904471https://www.genuki.org.uk/big/wal/CAE/Llandegai/Cas
Likely marriage - David & Mary married each other and can be found in censusMarriages Dec Q 1900 Evans William Bangor 11b 880 Owens David Henry Bangor 11b 880 Pritchard Mary Bangor 11b 880 Williams Maggie Bangor 11b 880 Cas