Have you attempted to find children to the marriages that you know of, and then looked for them on family trees on ancestry, Genes Reunited, etc?
Have you got the death certificates of his siblings, to see who the informant was?
I do have a lot of details about the descendants - I have deleted their names from the initial post as I was warned it was against forum policy to publish the names of the living. I have no idea where they are or who they married though. For example :-
1) William Parker , I have no idea if he married or not and where he lived etc as there are so many WPs in that era it's impossible to know. I have ordered the birth cert and hope that he will have definitely died after 1970 for then I can trace his death cert and see who the witnesses are. If he died before 1970 then there is no chance. The cert will take some time to come as I could not be sure of the GRO index so have asked for a general search with the parents' names.
2) Grace Ann Parker, married (Deans) but had no kids.
3) Thomas Oley Parker married and had 4 children, at least 3 of whom married themselves. One of them was a policeman in South Shields. I have traced at least 5 Grandchildren. 4 boys and 1 girl but I can't give any details obviously as they were all born after 1950 and so presumably still alive.
4) James Parker, 1908 I believe he lived his entire life in South Shields and died in 1982 but again I can't be sure so have sent for the birth cert with a general GRO 1 year each side search and will work forwards from that.
5) Isabell Parker, 1912 I know she was married (Errington) and divorced and had at least one son and lived her life in Sunderland. Again, can't list the name as he is presumably still laive.
6) Hannah Parker 1916 she lived in the same street as my Grandmother & parents. She was married twice, had two daughters , one granddaughter and two great granddaughters who would now be about 35-40 years old.
My Grandfather died in 1962, long before I was born. My mother tells me he never saw or talked about his father (William Parker 1874-1946) who died 15 years after my mother was born yet lived in the same town. He was pointed out to her once in the street by her mother when he was walking across the road. She recalls some of her father's siblings came to the funeral but being over 50 years ago, she can't recall who.
Of James, William and Isabell, she has no memory at all and did not know they even existed until I started the family research. She probably has between 10 - 15 first cousins who she has never ever even met or heard of which is quite sad in my opinion.
Sorry for the long winded reply
Similarly, of his 6 surviving siblings, she only remembers meeting and talking about an Uncle Tommy, Aunty Grace and Aunty Hannah.