Complex sentence: By the time Ardizzone returned to England in May 1945 he had completed almost 400 sketches and watercolours of the War, most of which, along with his wartime diaries, can be seen at The Imperial War Museum.
Split into simpler sentences: By the time he returned to England in May 1945 he had completed almost 400 sketches and watercolours of the War. An extensive collection of his war pictures, as well as his wartime diaries, can be seen at The Imperial War Museum.