One thing occurred to me last night that may place a bit of a spanner in the works, Joseph and Sarah were married in 1917, yet Mabel was born in 1911! Therefore if the Sarah are the same person it means Mabel was born prior to her registered parents being married! I do not know when John died tho a quick search showed someone matching the name and area died in 1910. I still think the two Sarah's are the same, the chances of exactly the same names seems just too much of a coincidence to me!
We know this was a time of turmoil for the family, John worked in the railway and had a severe head injury which made him very ill, we know Violet was in someway injured by John, her father, maybe Sarah sort solace with Joseph because she could not cope with Johns illness and Mabel was the result, and moved to Manchester together to escape the shame and then married when they could. This is just conjecture no real evidence although the is still one elderly relative still alive in Dudley in a nursing home though ill, have been meaning to visit now have a good reason maybe she can recall a few details which could help.
However I have established a blood relation between Thelma and my father which is what I wanted.
Mr Jones you asked about my grandfather Richard Moore, he was a Salford man born and bred, as you can see from the posts there is a Isle of Man connection with another side of family, Horrocks then Simpson then Bridson, it is possible Richard may have had connection which could be how Thelma met Marshall Bridson her husband who was from the Isle of man, I am not sure where Earlstown is, is it in London? If so we know of no connection.
The above has been very helpful and now I think I need to go to the records and thank you to all who contributed and help me on my way, will let you know how it concludes.