The Oxford Dictionary of Place Names has –worth as meaning an enclosure, hence enclosure of a man called Heppa.
ðɪ ˈɑːksfɚd dˈɪkʃənˌɛɹi ʌv plˈeɪs nˈeɪmz hɐz wˈɜːθ æz mˈiːnɪŋ ɐn ɛŋklˈoʊʒɚ, hˈɛns ɛŋklˈoʊʒɚɹ əvə mˈæn kˈɔːld hˈɛpə.