Lots here for you to think about Judy.
(I love Nancy Ninkavel as a name

)
Family Search has an extracted birth and baptism for Ann Nankivell 26th december 1826 and 7th Jane 1827 Tavistock, with parents James and Jane.
1841 has a really feint census with James and Jane (transcribed as James) living in ST Mary Tavy which fits with p o b however, I can't see Ann/Nancy - lots of children though.
If you want this I can post it.
1841 HO107; Piece 233; Book: Folio: 9; Page: 13
best wishes
heywood