I'm not sure if there is a St Agnes connection, but I will let you have info I have discovered so far on my family.
It starts with the marriage of JAMES NETTLE to MARIA SITTAR in 1807. They married in Ashburton, Devon. Maria's family, the Sittars or Sitters seem to have lived in Ashburton around that time, but I do not know where James was born. He was not alive by the 1841 census.
Their children (for definate) were JOHN NETTLE (my GGGGGrandfather) born Ashburton in 1810 died in Caharrack, Cornwall in 1876 and Maria born 1811 also in Ashburton.
There are later births to a JAMES NETTLE and MARIA in St Agnes from around 1818 namely William, Eliza, Maria, Eilzabeth and Anna but I'm not sure if these are the same couple. It's possible the family moved back to their father's parish in St Agnes but I just do not know for sure. Mining families quite often moved around as new mines opened and old, unprofitable ones closed.
There is also a James Nettle and a Mary having children in St Agnes at the same time but I am confident these are not the same couple with a variation spelling of Maria because of the times of the births.
By 1841 Maria Nettle is a widow living in Poldice, Gwennap (Redruth area of Cornwall)and my Nettles seem to have stayed around the Redruth area until the turn of the 19th century.
If you have details of a James Nettle born (I guess) around 1785 and cannot place him, please let me know - he might be mine!!!