Hello We.......
In 1841 John appears to have lost his Mum is that correct? his father being William the Shoemaker, and his parents are William and Elizabeth?
1841 census
Bay Tree Hill, Liskeard, Cornwall
WILLIAM JAMES 60 AG LAB Y
ELIZABETH " 55 Y
WILLIAM " 28 SHOE M Y
WILLIAM " 12 Y
JOHN " 5 Y
MARY ANN " Y
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and in 1871
1871 census
7 Wormington Road, Kensington
John H James 40 carpenter Cornwall ?
Mary " 30 laundress Bath, Somersetshire
Emma " 10 St Pancras, Middx
Florence " 8 "
John H " 5 "
William " 3 Hammersmith
George " 9 months Kensington
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Now if I was you I would find one of the last births ie George born 1870 SEPT QTR, 1870, KENSINGTON, VOL 1A, PAGE 106 on freebmd and put in a request for Christmas.........one birth cert cost £7-00 online which will give you mother's name and Maiden name as well as father's name and occupation. The mother's maiden name will enable you to go looking for the marriage date from another angle.......
*** As Yet........As the census is to the head of the family.......you have no firm evidence that Mary is the mother of any of the children in the 1871 census!
My suspicions are that she is the mother of the last three but is she the mother of the first two?
Q......was Emma or Florence around in 1881........someone has to fill in the census sheet, the enumerator picks it up and transposes it!.............now if mum and dad don't read and write too well who would fill in the sheets.........only their was a
EMMA JANE JAMES born SEPT QTR 1861, PANCRAS, VOL 1B, PAGE 171.
would her real mum have been JANE......................fantasy, probably?..............
Q......How do yo know that JOHN H JAMES is John Henry James
In summary.........the purchase of one cert (George 1870) should be the start of your research and will lead to the answer of all your questions?

Regards
Bryant