On 26 January 2015, Hugh Grant entertained an unusual guest at an exclusive venue in one of London’s most affluent neighbourhoods. A few weeks earlier, the disgraced former tabloid journalist Graham Johnson had been contemplating starting the year behind bars. Now, he found himself opposite the Hollywood actor in the rather more comfortable surroundings of the KX Gym in Chelsea, which doubles as a private members’ club where fees cost more than £600 a month. It was on that day, 11 years ago, that one of the seeds of Prince Harry’s doomed court battle with the publisher of the Daily Mail was sown. The privacy action brought by the prince and six others, including Elton John and his husband, David Furnish, the actor Elizabeth Hurley and Doreen Lawrence, would probably never have made it to the high court had it not been for an unlikely alliance. It was one forged between Johnson – a self-confessed “professional liar” who had regularly fabricated stories for the tabloid press – and Evan Harris, a former Liberal Democrat MP …