I took my information from a Snewin genealogy document, which has some inaccuracies. I'll see if I can follow up for more details.
On closer inspection I see no record of a marriage of John Snewin and Hannah. Was it possibly a common-law arrangement? If Hannah married Thomas Jackson in 1842, is it possible Jackson died in the 1840s?
Do you have the birth cert. for Harry Snewin (Harry Thomas Snewin b. 3Q 1850)?
John Snewin was christened 22-Oct-1815, St John Hackney, son of Philip and Ann. I have a screen shot of the birth record, and it's available on ancestry.com.