Where Was John Curtis Born?

John Curtis appears on the 1841 census living with wife, Ann, with their family in Sutton St Mary, (Long Sutton), Lincolnshire.
(John Curtis is the 5th great-grandfather of my grandsons, Kian and Caleb Curtis.)

In the column where it says 'born in Lincolnshire' both John and Ann answer 'NO'.
Unfortunately, John died before the 1851 census, so I haven't been able to work out where exactly he WAS born, though this would have been about 1790.
Better luck with wife Ann, she appears on several censuses, which all state she was born in Hadley, Enfield, Middlesex, around 1789. On later censuses she states her occupation as 'retired public house keeper'.
I haven't yet been able to find Ann's maiden name, or their marriage. Although they started having their children in 1824 whilst in Sutton St Mary, I have checked back of the Sutton parish records and they definitely weren't married there!
Another small clue I have is that on son Charles' baptism record in 1824, John Curtis is described as a LABOURER, but on his next child's record in 1826, he's described as a MALTSTER. Could this mean that this was when they began their inn-keeping trade? Also, because John was described as a maltster and not an innkeeper, could this imply that the public house trade was passed down via Ann's family?

Much more work to be done on this, but I seem to have hit the old brick wall!