Complex sentence: Admiral Sir William Reginald Hall ("Blinker Hall") was British Director of Naval Intelligence from late 1914, though he was only made a Rear Admiral three years later, and during the timescale of the film would have held the rank of Captain.
Split into simpler sentences: Admiral Sir William Reginald Hall was British Director of Naval Intelligence and was refered to as either Admiral Hall or as'Blinker Hall' during the film. Though he was indeed head of Inteligence from late 1914, he was only made a Rear Admiral three years later, and during the timescale of the film would have held the rank of Captain.