There is a long list of Preece baptisms at Dilwyn, including Francis son of Hannah in 1792. Looked at Counties in England on the Hugh Wallis link given. John is down as son of Francis and Elizabeth and his son born in 1840 is Edward, mother Sarah.
Worth checking everyone one of these and plotting them as there is a probability of more children for Francis and Elizabeth. A birth Certificate would be available for little Edward born 1840