This is a reference to Helen Duncan (1897-1956), who was imprisoned under the Witchcraft Act in 1944, the second last such case in Britain.
In November 1941, Duncan held a séance in which she described a dead sailor wearing a cap marked HMS Barham, who told her his ship had sunk. This caught the attention of the authorities, as the HMS Barham had indeed been sunk, though this had been kept secret from the public. Duncan's activities as a medium are known to have been fraudulent. Her reference to the sinking was probably an attempt to gain credibility by revealing unknown information. Her source for the intelligence is unknown. The government, afraid that she might have access to details regarding the D-Day invasion of Normandy, managed to secure a conviction under the Witchcraft Act, which prohibited fraudulent 'spiritual' activity.
Duncan was released in 1945 after she promised to stop holding séances, though she continued to do so and was rearrested in 1956 for violating this condition (the Act itself had been repealed in 1951). She died shortly afterwards. A campaign for her posthumous pardon is ongoing.