Hart married Jenifer Fischer Williams, a civil servant, later a senior civil servant, in the Home Office and, still later, Oxford historian at St Anne's College (specialising in the history of the police). Jenifer Hart was, for some years in the mid-1930s and fading out totally by decade's end, a 'sleeper' member of the Communist Party of Great Britain. Three decades later she was interviewed by Peter Wright as having been in a position to have passed information to the Soviets, and to Wright, MI5's official spy hunter, she explained her situation; Wright took no action. In fact her work as civil servant was in fields such as family policy and so would have been of no interest to the Soviets. The person who recruited her, Bernard Floud, interviewed by Wright shortly after, maintained that he was unable to remember ever having done so. Nor was her husband in a position to convey to her information of use, despite vague newspaper suggestions, given the sharp separation of his work from that of foreign affairs and its focus on German spies and British turncoats rather than on matters related to the Soviet ally. In fact, Hart was anticommunist.