Hello, Dave,
A bit of a long shot, but have you considered the possibility that James ( wife Emily) could be related to Joseph ( wife Sarah) ? This could account for a Samuel Appleton living with James & Emily as as boarder, not a relative ie., brother in law. You know Joseph's wife Sarah's maiden name was Appleton, so is Samuel related to her in some way.
Sarah E.Appleton was a servant in the 1881 census St Paul, Deptford born 1868, Holloway.
In the 1871 census she is with her parents..
1871 census
RG10/305 folio 86 page 10
2, Hercules Pl, Upper Holloway, Islington
Moses Appleton age 28 Carman b Essex, Colchester
Mary Ann wife age 26 b BEDS looks like Cedpham ?
Thomas son age 4 b Islington