The marriage of John Niblett and Sarah Hart took place at Westbury on Sept 20, 1802. Cornelius was christened at Westbury on July 17, 1814. He moved to Gloucester.
In the 1841 census Cornelius is not recorded but John and Sarah were living next door to the Harts with their son Anthony and his family at Ampney. Anthony was christened on Nov 28, 1802. Rachel was christened on Aug 18, 1805 and Edmund on May 5, 1817.
I do not believe that Samuel was the son of that John and Sarah. The birth was before their marriage by 11 years. John was born in 1775 which would have made him 16 at the date of Samuel's birth. Westbury was an area where illegitimacy was not uncommon but that seems too much of a stretch and Sarah was born in 1781 (1791 in the 1841 census but this must have been an error)so Samuel must have been born to another John and Sarah.
The likelihood is that this was John Niblett and Sarah Taylor who were married at Westbury on Sep 16, 1771. They had two other children Thomas B. 1773 and John B. 1775. The latter John was, I believe, the John who married Sarah Hart.
Although there is a 16 year gap until Samuel was born this was not without precedent.
Naomi, I suspect we have corresponded on a similar subject so if you want to take this further you could e-mail me direct.