Hi Eileen, thanks for all your good work.
Yes, John Abraham and Emma are my most likely candidates for his parents, but I cannot prove it. There is no entry in the BMD index for such a birth. My one hope is that there is a local Essex, West Ham or walthamstowe record that was not listed in the national index.
There are lots of John Daveys about. The 1872 ones are too young to be him (b1869). John and Emma and family do drift in and out of the census. It seems they lived for a number of years in the Forest Gate area and appear in one census ( 1881 I think) as John Abraham Darcy. I have tried to follow up on his sisters, but without success so far.