Spelling was optional, back then, and the same name might be spelled several different ways. "Hammon" is one of the less likely ones, but I still would not be surprised to find it occasionally.
More common even than "Hamon", especially the earlier you go, is "Hamo". Various charters and other legal documents have been found with this form. http://discovery.nationalarchives.gov.uk/details/r/C9544520 (c. 1329)