UPDATE
I did get the birth certificate of Eleanor's son, Charles Butler: he was born on 13 December 1888 at 38 Warrender Road, Islington; father: BLANK; mother: Eleanor Frances Butler, formerly Greene, of Independent Means; informant: E F Butler, Mother, of 38 Warrender Road, Islington. I can't find someone with this birthdate, for example, on the 1939 Register?
I can find no obvious link between Eleanor and the occupants of 38 Warrender Road, who are listed on the 1881 and 1891 Censuses?
FURTHER UPDATE!
Captain George Butler married his "good friend's" (Charles Greene's) daughter (Eleanor) in 1881. However, Captain Butler is shown on the 1881 Census with his WIFE (Harriett Ann, née Cozens) and daughter at 367 Victoria Park Road, Hackney.
When he died on 26 March 1884, it was his WIDOW, Harriett Ann Butler of 367 Victoria Park Road, who applied for Administration, not his "WIDOW" (as mentioned in the newspaper articles reporting on his death), Eleanor Frances Butler.
So, Captain Butler married Eleanor bigamously. Surely his good friend, Eleanor's father, must have known that he was already married, with children?
FURTHER UPDATE AGAIN
There is a marriage on 3 October 1918 between Eleanor Frances Butler (spinster) and Daniel Patrick O'Hanrahan. However, she gives her father's details as "John Butler (deceased), cabinet maker".
When this couple appear on the 1921 Census at 27 Inniskilling Road, West Ham, her birthplace is given as "Berkshire Windsor", ie not that of Eleanor Frances Greene - St Pancras.
BUT the new Mrs O'Hanrahan doesn't seem to have existed as Eleanor Frances Butler, with a birthplace of Windsor around c.1863, or appear on any Census with a father, called John?
I know Eleanor Frances Greene-Butler's tendency was to be very generous with the years she knocked off her age (8 years at the 1911 Census); and she could quite possibly have maintained only a 2-3 year difference between her and Daniel for appearances sake, but with all the other information not matching up, I am struggling to think Mrs O'Hanrahan is the right Eleanor Greene.
If I could find something that shows Mrs O'Hanrahan was a separate person, that would be great - but obviously would leave me still with the missing Eleanor Frances Greene-Butler!
Perhaps in all this, I've missed something fairly obvious?