There is a Mary ENFIELD age 60, not born in County, living a few doors away from William in 1841 at John Street, Ordnance Place, Chatham. I would think this is his mother.
In 1851 Census they are living next door to each other as follows:
HO107/1610 F 459 P 9
19, John Street, Ordnance Place, Chatham Mary ENFIELD Head Widow aged 66 Carpenters Widow b Polsted, Suffolk 18, John Street, Ordnance Place, Chatham William ENFIELD Head aged 30 Carpenter Sarah ENFIELD Wife aged 28 George ENFIELD Son aged 8 Sarah ENFIELD Daughter aged 6 Mary Ann ENFIELD Daughter aged 4 Herbert ENFIELD Son aged 1 All born Chatham