Hello again Susan,
As promised, I have been digging around and have uncovered the following:
In the 1871 Census (in April 1871):
John Seager, Head, b abt 1823, Upchurch
Frances Seager, Wife, b abt 1807, Rodmersham
Eliza Newman b abt 1862, Rochester, described as the Granddaughter of the Head ie of John Seager (but, I suppose, equally of Frances). So:
John Seager and Frances Castle marry Q4 1865 in Milton Registration District.
Frances is listed as Frances Castle, Widow, in the 1861 Census at Lower Halstow, with one child, Jesse, b abt 1843.
In the 1851 Census Frances is with her first (?) husband, Samuel Castle and three children: Daniel b abt 1834, Eliza, b abt 1840, Jesse b abt 1843.
The marriage of Samuel Castle and Frances [Surname?] is probably before 1834 and therefore before civil registration. Thus need to search Lower Halstow and parishes around for the marriage.
In the 1861 Census:
John E Seager, Head, b abt 1824, Milton
Ann Seager, Wife, b abt 1827, Teynham
George Seager, Son, b abt 1848,Bobbing
Sarah Seager, Daur, b abt 1850, Borden (?)
John C Seager, Garndson, b abt 1860, Milton
Sarah A Thomas, Visitor, b abt 1859, Ospringe
John Edward Seager Marries either Emily Longley or Mary Jarrett Q2 1843, Milton ????????? (My bet, for what it is worth is that it was Mary Jarrett as Mary Ann was a common name but of course I could be wrong!)
Where this leaves the relationship between John Seager and Eliz Newnham I can't for the moment figure out! Need to find out Frances' birth surname to throw further light on this.
(NB: Frances is also spelt Francis but there is little doubt that she was female - her age is in the female column.)
I'll see what I can find out.