Oh wow!
It looks like John remarried late in life.
John Cornwell married Sarah Shipp Cambridgeshire, Newmarket 1848 Q2 volume 14 page 139
We then see them in the 1851 Census:
Vicarage, Swaffham Bulbeck, Newmarket, Cambridgeshire:
John S Cornell, 80, Retired Farmer, b. Carlton, Cambs
Sarah Cornell, 56, wife, b. Swaffham Bulbeck
Eliza Blanks, 20, house servant, b. Little Wilbraham
The death is more problematic. He does not appear in the 1861 census; but there are four possible deaths, all registered as John Cornell in Cambridgeshire, Newmarket:
1855 Q2 volume 3B page 375
1856 Q1 volume 3B page 325
1859 Q4 volume 3B page 357
1861 Q1 volume 3B page 359
His widow Sarah is to be seen in the 1861 census:
Grosvenor Yard, Church lane, Newmarket St mary
Mary Cornwell, widow, 34, Needle Woman, b. Great Bradley, Suffolk
Joanna Cornwell, daughter, 12 Scholar, b. Kirtling, Cambs
Ben Cornwell, son, 6, scholar, b. Newmarket, Suffolk
James Cornwell, son, 4, scholar b. ditto
Henry Cornwell, son, 2, b. ditto
George Cornwell, son, 0, b. ditto
Sarah Cornwell, mother in law, 64, formerly laundress, b. ditto
(I suspect this was a case of Mary not knowing where her mother-in-law was born, and so just entering the place they happened to be on the return)
I've had little joy so far in tracing Mary back to a marriage and a husband.
There are three promising deaths which could be her husband:
George Cornwell, Cambridgeshire, Newmarket 1860 Q2 volume 3B page 359
Henry Cornwell, Cambridgeshire, Newmarket 1860 Q2 volume 3B page 363
Edward Cornwell, Cambridgeshire, Newmarket 1861 Q2 volume 3B page 339
I cannot, however, find any marriages that would tie up or 1851 census entries to tally. (If forced to choose a favourite, I would plump for George, simply on the basis of the name given to the posthumous son ... but I know that's pretty thin).
In the 1871 census I think that she is probably Sarah Cornwill, widow, 78, retired farmer's wife born Cambs, Lode who is living on her own at Mill Road, Bottisham, Newmarket
Finally, I think she is probably the Sarah Cornwell, age 95 (!), whose death was registered Cambridgeshire, Newmarket 1875 Q1 volume 3B page 466
If she was living alone, then it may well be that whoever registered her death merely thought of her as "a very old woman" and had to guess at an age - so I am not too troubled by the age. She is not in the 1881 census, and there is no other plausible death registration.
Well ... there's an AWFUL lot of information to process there.
Hoepfully some of it will help me to unscramble this line a little further.