For decades Henry Allingham shut out his wartime memories. When he died, at 113, he was the oldest man in the world, and the oldest surviving volunteer serviceman of the First World War. And yet, in the closing stages of an extraordinary life, Henry decided to tell his story as a tribute to the millions who never returned home. BBC reporter Robert Hall spent four years travelling with Henry as the last volunteer battled age and ill-health to do his duty, and remain on parade.