Complex sentence: After the war, when the WAAC collection of artworks was dispersed to different museums, the complete "Shipbuilding on the Clyde" series was offered to the National Maritime Museum who refused to accept the pictures and they were given to the Imperial War Museum instead.
Split into simpler sentences: After the war, when the WAAC collection of artworks was dispersed to different museums, the complete "Shipbuilding on the Clyde" series was offered to the National Maritime Museum. However the National Maritime Museum refused to accept the pictures and they were given to the Imperial War Museum instead.