Hart's war work took him on occasion to MI5 offices at Blenheim Palace, family home of the Dukes of Marlborough and the place where Winston Churchill had been born. He enjoyed telling the story that there he was able to read the diaries of Sarah Churchill, Duchess of Marlborough, wife of the founder of the dynasty John Churchill, 1st Duke of Marlborough. Hart's wit and humanity are demonstrated by the fact that he particularly enjoyed the passage where Sarah tells that John had been away for a long time, had arrived suddenly, and "enjoyed me straight way in his boots". Another incident of life at Blenheim which Hart enjoyed recounting was that he shared an office with one of the famous Cambridge spies, Anthony Blunt, a fellow member of MI5. Hart wondered which of the papers on his desk Blunt had managed to read and to pass on to his Soviet controllers.