Sometimes it's the doors. To complete a full circuit of the museum more than 3,000 doors need to be opened and closed. Some of these, particularly ones that seal off the major galleries, are cumbersome to shut. But when bolted, they won't open again without a tussle. Except when they do. Take the Sutton Hoo gallery, which houses treasures from an Anglo-Saxon ship, among them a ferocious-looking helmet believed to have been worn by Raedwald, king of the East Angles, in the seventh century. On one occasion a guard bolted the double doors and moved on to the next room,...