Looking at the 1921 Census for 25 Balmes Road, Location London ...
Martha Brooks [nee Hall] was the Mother of Joseph T Brooks, born Hoxton.
I'm pretty sure that the RN serviceman Joseph BROOKS (turned furniture polishe/salesperson etc) is the same man who married Martha HALL ...
and then ended up an ailing RN pensioner with Mrs HARMER, widow in 1921.
Sue
1911 Census
Looks to have been found previously due to the Shop Assistant, Furniture, reference for Joseph Brooks, born Bermondsey.
1911
46 Halliford Street, Islington
Joseph Brooks, Head, 36, M, Shop Assistant, Furniture, Bermondsey.
Martha Brooks, Wife, 43, M, ....
With children
Doris Florence Brooks, Dau, 9, Islington
Thomas Joseph Brooks, Son, 7, born Hoxton.
Edward John Brooks, Son, 5, Hackney.
Beatrice Lilian Brooks, Dau, 4, Islington.
-----------
The Navy Index Card indicates this 1911 couple have split up with Martha Brooks [nee Hall] at 25 Balmes Road by 1921.
Joseph Brooks by 1921 Census is at 47 New Road, Chatham, with Mary Elizabeth Harmer, a Widow, listed as a Servant.
Mary Elizabeth Harmer, appears to be nee Sayles, See Marriage page 7 Reply 54 & Quote.
Mary Elizabeth SAYLES married Albert Alfred HARMER 5 Nov 1899 in Willesden, London but don't know if this is any use?
Marriage image online - https://www.ancestry.co.uk/discoveryui-content/view/7800137:1623
Mary Elizabeth Harmer (nee Sayles) widow, occupation is a Dresser line showrooms, Furnisher in 1911 - maybe how they met?
1911 - https://www.ancestry.co.uk/discoveryui-content/view/39837601:2352
I wonder if this is Mabel Ethel? Why would she change names on BC but give her maiden name...
Cas
The man that Mary Elizabeth Sayles married appears to have died in the Registration District of Hendon.
1907
Albert Alfred Harmer, 30 years, died in the Reg'n District of Hendon.
------------
I agree with others regarding seeing the Margin Note in full to see what was changed on the Death Certificate.
Mark
Added: I forgot so say that my suspicion is that Sayles at the Harmer=Sayles, Marriage might be related to Sayles on the 1915 Birth Certificate?
Which has already been suggested.
But one can find coincidences occasionally.