Hi again @Linray,
I'm afraid I don't have the census return from 1921 for your father, only a transcript of it as I access the 1921 census through a friend. I'll send the transcript to you but it also contains a few of my 'scribbles'. From info gleaned from my friend, your grandmother, Jane Isabella, seems to have married (Lavery, living Stakeford) and in 1921 has one daughter, Daisy. She thinks that Mary (full name Mary Alice Cropp Giles) was adopted by a couple in Blyth, surname Francis. 'Cropp' is probably the father's name just as Forster is the father's name in the Giles family. This was very common practice. You will see on the 1921 census transcript that even Mary Ann Giles has had a name added - Cavagin - and this is probably her father's name. This should give you a few streets to go down in your search. I'll scan the 21 transcript and get it off to you some time today.