There are two possibilities who your Mary Daffern was.
1.The daughter of Joseph Daffern and Hannah Leech, married in 1817, a daughter, Mary baptised in 1826 at Nuneaton.
2.The daughter of John Daffern and Elizabeth Petty, married in 1820, daughter Mary baptised in 1825 at Stockingford, Nuneaton.
I have eliminated the first one as she was living with her parents, Joseph and Hannah, as an unmarried daughter in 1851 census.
So it is quite possible that your Mary was the daughter of John Daffern and Elizabeth Petty.John, baptised in 1789 was the possible son of William Daffern and Elizabeth Brook married in 1787. William was baptised in 1758, the son of Eustace Daffern.Eustace was baptised in 1729, the son of Richard Daffern and Sarah Spencer married in 1727.
The only way to prove that Mary was the daughter of John and Elizabeth is to look at a copy of the marriage certificate where the father's name is given.