Family search have a William Hardy baptised 25 January 1708, father John, mother Thomasine, in Elsing.
A fair way from Caston but in my experience of OH's Norfolk ancestors, they travelled about the county a surprising amount - large families in small villages with not enough jobs so the gamekeepers/blacksmiths/ag labs/domestic servants had to travel to find work. I made the mistake of only looking at neighbouring parishes at first!
Good luck
Pat