From the National Archives website:

From 1920 to 1923 he headed a civilian mission to Japan engaged in training Japanese aviators. He maintained close touch with Japanese missions and officials throughout the 1920s and 1930s. Before and during the war he was associated with a number of Fascist or semi-Fascist individuals and organisations, including ‘Action’, ‘The Link’, and the ‘Constitutional Reform Association’. There is circumstantial evidence that in the1920s and 1930s Sempill passed sensitive military and technical information to the Japanese. For reasons touched on but not fully explained, Sempill was not prosecuted under the Official Secrets Acts, though this was seriously considered in 1925. On the evidence of these files, Sempill’s activities on behalf of the Japanese and his Fascist contacts were motivated less by any desire to help the enemy or to make money than by his own impetuous character, obstinacy, and flawed judgement.