Dear Sylvia,
Thank you for your useful information and I believe another piece of the puzzle has fallen into place because of a small part of it.
We know little of Susan Montgomery, other than she was Susannah Webb's daughter, married Robert Montogomery, later surfaced in London and possibly had a child surnamed Harding, which may imply a second marriage.
She was known as Aunty Dot and the family tale is she ran a business (pub?) somewhere in the Elephant and Castle area, possibly sourcing another pub for her brother Samuel Webb, my grandfather, who moved to London with his wife May to run one about 1921.
It would seem from your information today that her mother, Susannah Webb nee Bridson, the lady for whom we are seeking information, was living with Susan in Douglas in 1911.
However, after that date we know nothing - but you mention a possibility, a death record which would need a Liverpool connection.
Susannah's other daughter was Ellen Sephton Edgar, Susan's sister. She was known to the family as Aunty Nell and died in 1940, together with her husband when their house in Liverpool was destroyed by a bomb. Their daughter, Jean, miraculously survived, but was deeply affected by the experience and passed away in the 1960s, we believe.
It is worth surmising therefore that Susannah Webb relocated at some point to Liverpool to be with / near her other daughter, perhaps rather than going to London or being left alone on the Island, dying in 1935 in her 70s.
More research required then, but your information has been most useful, many thanks.
Regards
Peter