The Dennett name was carried to England during the migrations from France following the Norman conquest in 1066
The Dennett family lived in Sussex. The family was originally from Anet in Normandy and it is from this location that the surname derives.
See Wikipedia Chateau D'Anet
Before English was standardised French, Latin and other languages had an influence on the spelling of names even among the literate.
Other spellings include: DENNET, DENET, DENNET, DENNETTE,
American sources and Surname studies suggest that Hugh d'Anet went from Normandy to Sussex with William the Conqueror, and had large estates settled on him ( including Hurstpierpoint and Woodmancote). There are archive records (3825/D460) of almost 300 title deeds.