In May 1968 Lord Mountbatten met Cecil King, Hugh Cudlipp, and Solly Zuckerman to discuss preparations for what King saw as an inevitable collapse of law and order as a result of Wilson's weak and socialist Goverment. Mountbatten was to lead a National Goverment in a sort of Ministry of All-Talents, however when Zuckerman said it would be treason Mountbatten declined.
King would write some headlines calling for Wilson to be removed, however this would result in him being removed later that month from his position as head of the IPC.
MI5 comes in a bit later, but the gist is that, had Mountbatten accepted, he would have been in the shadows to form a Goverment if a Constitutional Crisis happened, which MI5 is alleged to have been trying to engineer a destabilization of the Labour Goverment, as had the Secret Services done in 1924. Basically if the coup was put in place, it would have happened in the 1970's.