I think you have almost certainly encountered a faint echo of the case of "Hellish Nell," the medium Helen Duncan, who was in 1944 prosecuted under the Witchcraft Act of 1735 for the crime of fortune telling – something more usually dealt with under other the terms of other legislation, such as the Vagrancy Act of 1825.

Duncan, who was found guilty and sentenced to 10 months in prison, was not a witch in any conventional sense; she was a spiritualist. Her real crime appears to have been to have claimed to have summoned the spirit of a dead sailor who had served on HMS Barham, a British battleship which was torpedoed and sunk in the Mediterranean in 1941, and to have made public the ship's sinking before the Admiralty had admitted that she had been lost, causing a significant problem for morale in the major naval base at Portsmouth when word of the disaster got out. There were other, similar, but less dramatic leaks of privileged or secret information at other seances later. It seems to have been for this reason that she was prosecuted under the fiercer provisions of the Witchcraft Act.

As to where Duncan got her information – if not from the spirit world – that remains a mystery, but it is assumed she must have had some contact among the personnel at the base who was privy to information about both the Barham's sinking and other incidents.

Malcolm Gaskell, a noted historian of early modern witchcraft, wrote a good book on Nell's case if you are interested in finding out more.