Hi again
Always frustrating when that elusive baptism doesn't want to be found. A few miles radius from Caston would definitely be reasonable maybe even further depending on occupations. Some of my Suffolk ancestors moved quite a distance into Norfolk presumably where there was more work. In the parishes you have checked out so far, were there any Hardy baptisms? If so, it's just possible that William's may not have been recorded, not unusual in those days. I try and check the parish records, BTs and ATs just in case there is anything different. If there are Hardy parents baptising children, do the parents names seem to follow through in William's children? Difficult trying name association with really common names like John, but Michael might be more of a clue.
It's a bit of a jigsaw and often some pieces stay missing - annoying!
Shirl