Taking a step back here:
George & Fanny Green baptised the following children in Campton
Fanny 16 Feb 1806, born 12 Jun 1803
William 16 Feb 1806, born 3 May 1805
Elizabeth 10 July 1808
George 18 Feb 1810, born 12 Jun 1809
George, no parents named, was baptised on 10 Jul 1808, the same day as Elizabeth, so I think it's reasonable to assume that it was an adult baptism of George the father of Elizabeth.
Which makes it all the more vital to establish his age on burial ie is the 1840 burial that Jan found the correct one, in which case we're looking for a 15 years older George than we thought.
What baptism/birth dates and places do you have for the other children of George and Fanny?
John and Martha baptised a number of children in Campton 1770-79. Might George have been their son, who somehow got missed?
On an earlier thread,you were given a marriage of John Green and Martha Neal née Green at Campton on 19 Nov 1767. Whilst this isn't on the IGI it looks OK, and there's time for George to have been born 1767-69.
Given that at least one Campton marriage seems to have been missed from the IGI I'd be tempted to view the microfilm to see if others may have been similarly missed
David