There is another census record 1871 and beyond for John and Elizabeth of Lea, Glouchester - I couldn't find their Mary after that, which indicated she possibly left home to work, or married.
Brenda - I feel this Mary James can be ruled out as father in law was staying with the James family in 1871 his surname CAUDLE, marriage
John JAMES name on same page
Elizabeth CAUDLE 1853 Ross
Looking at birth entries on FreeBMD for London, Middlesex & Surrey these came up, trying to see if I can tie any up with
parents named John & Elizabeth JaneMary James births between 1859 – 1861 (FreeBMD)
Shoreditch Mar 1859
Lambeth Jun 1859 - established from bapt. Mary's parents are
John & Mary AnnKensington Jun 1859
Camberwell Mar 1860 (middle name JANE)
Clerkenwell Sep 1861
I'm was looking into the birth entry below born in WALES not ENGLAND

only because of the name John - haven't located them on the census to check parents names though
Mary James & John James
Jun 1860 vol 11b page 105 Brecknock
There’s an asterick by district for Mary James but looking at the original entry on FreeBMD it reads
11b 105 as does John’s
looking at a possible marriage which may tie in mmn Jenkins

1851 Brecknock John James name on same page Elizabeth JENKINS
Have you followed Mary's Aunt Susan Edwards on any UK censuses to see if that gives you a clue?
Do you know where she was born?
As previously posted this is the only marriage I can see for London area for a John James to Elizabeth Edwards as possible parents for Mary if she was born in London
Mar 1860 Islington
John
JAMES Names on same page
Elizabeth
EDWARDSEugene Napolean Lefort
Elizabeth Cook
Looking at the census for Eugene Napolean Lefort his wife Elizabeth born 1839 Bedfordshire
checked 1851 census there's an
Elizabeth COOK born 1839 Bedfordshire
Although not 100% certain I would think that John James has married Elizabeth Edwards
a shame the marriage cert. is not on line to view
