The art is tucked away on the second floor, arranged in a series of permanent galleries and an exhibition space devoted to temporary shows plumbed from the collection. The latest of these is "Bosnian Landscapes and Shipbuilding on the Clyde", two distinct and separate bodies of work made by Stanley Spencer in the aftermath of the First and during the Second World War.
Spencer is a popular artist, our closest thing to a national favourite for the 20th century, and yet there is not a single noticeable sign in the museum to alert the public to the exhibition's existence. I spent an hour there yesterday without seeing another soul. It seems a sad and rather shameful waste of the museum's efforts and resources. Sadder still because it's an excellent exhibition.
It's a show in two parts: opening with Travoys Arriving with Wounded at a Dressing Station, the only picture that Spencer painted as an official war artist in the First World War, depicting the wounded arriving at a makeshift hospital in the Macedonian countryside, an event that he remembered from his active service in 1916. It's a formal picture, carefully composed from memory three years later; successful in a documentary way, it lacks the quirky originality that marks Spencer's later work.
In this exhibition Travoys serves as an introduction to the "Bosnian Landscapes", a group of seven pictures painted on Spencer's return to Macedonia in 1922 and presented here as a link between his actual experience of war and the work that was eventually inspired by that experience: namely the Sandham Memorial Chapel at Burghclere.
These landscapes and a couple of early sketches for Burghclere have been borrowed from here and there, but the real reason for this exhibition is a chance for the IWM to show its own holdings of Spencer's work from the second war - a single and, as it turned out, giant commission to record the war effort at home in "Shipbuilding on the Clyde".
Taken together, these eight enormous canvases have a heroic quality that belies the ordinariness of the subject: a community of workers in brown tweed and cloth caps, welding and riveting their way to victory. Among them, hunched in a corner, the figure of Spencer himself: "I like it here," he wrote, "being lost in the jungle of human beings, a rabbit in a vast rabbit warren."
The two parts of this exhibition make perfect sense individually - one painter, two wars - but, as an exhibition of Spencer's work, the effect is a little odd. It's like a sandwich without any filling. The two halves are divided by a gap of 20 years, which in Spencer's case were the years in which he emerged as the painter we know and love: a sexually obsessed eccentric with a highly personal and engaging vision of the world. Here he seems an altogether more conventional figure: it isn't a true picture, but it's a good show none the less.
The Imperial War Museum, Lambeth Road, London SE1 (0171-416 5000) to 26 May