My quest is looking even more likely in that I'm much closer to convincing myself that William Edward AUSTEN's father William Mallars Fitz AUSTEN / Fitsaustin is one and the same as my William AUSTIN/AUSTEN married 1853 St Martin in the Fields to Emma Clinton PARKER.
The updates:
The marriage cert. in the Register Office, Whitechapel (7 Aug 1869, not 17th as per excerpt published on this or the other related thread being continued here) of William AUSTEN (as signed) AUSTIN (as indexed) to Sarah BIGGS nee BLAKE shows both as widowed, William as 40.
Widower not being entirely accurate, although wife number one had remarried, as a spinster, in 1855.
William's death cert. (27 Oct 1905 Hackney Union Infirmary, Homerton) shows: cert. William AUSTIN as 74, railway goods porter "of 62 Crozier Tce, Homerton". Died of senile decay, chronic bronchitis; Informant J W Oliver, Assistant Superintendent, Hackney Union Infirmary, Homerton; Reg, 3 Nov 1905
Another DNA match has popped up between my lot, and a descendant of another line down from William Edward AUSTEN and Alice Maud CARTER, which pinpoints this couple as "of interest" in the original tree. We have not yet ruled out the Carter ancestry so might be guilty of confirmation bias here!