HI,
there isn't much doubt as to your John's date of birth. There were a number of siblings too. One brother was Charles, he eventually ended up in Milborne Port and his daughter Kezia married John's son Henry. Robert Hann moved into Yeovil itself with his wife and family, he is on the 1841 census. However we now know that they moved to London before 1851 and I have Ann's death cert witnessed by Robert's sister. My Ancestor was his son Frederick who also went to Milborne Port.
I have done a lot of research on this branch of the Hanns and would be happy to share. There are a number of researchers looking at the Thornford branch, mostly descendants of the brothers who went to Australia.
Kindest regards - Jean