[7] Beginning in 1919 he attempted to absorb the counterintelligence service MI5 into the SIS to strengthen Britain's efforts against Bolshevism, an idea that was finally rejected in 1925.

[9] Sinclair was asked in December 1938 to prepare a dossier on Adolf Hitler, for the attention of Lord Halifax, the Foreign Secretary, and Neville Chamberlain, the Prime Minister.

[10] In the dossier, which was received poorly by Sir George Mounsey, the Foreign Office assistant undersecretary, who believed that it did not gel with Britain's policy of appeasement, Sinclair described Hitler as possessing the characteristics of "fanaticism, mysticism, ruthlessness, cunning, vanity, moods of exaltation and depression, fits of bitter and self-righteous resentment; and what can only be termed a streak of madness; but with it all there is a great tenacity of purpose, which has often been combined with extraordinary clarity of vision".

On 29 October, Sinclair underwent an operation for his cancer and died on 4 November 1939, aged 66, five days before the Venlo incident.

[12] Hugh was the son of Admiral Frederick Beauchamp Paget Seymour, 1st Baron Alcester[13] and Agnes Sinclair.