Looking at Elstob births from 1900 I found that they came exclusively from a small area of north-eastern England, and there appeared to be several Elstob families having children in Sunderland.
From 1911, when mothers' surnames were mentioned in the BMD records, I found other children of Joseph and Maria - Mary J Jun Q 1912, Catherine A Sep Q 1914, David B Dec Q 1920, Jane K M June Q 1924 and of course Florence B in June 1926. There were other Sunderland Elstob births between 1904 and 1911, but of course no mother's name to cross-check whether they belong to Joseph and Maria.
Gillg