You won't be surprised to hear that the puppet hero from the stage production of War Horse is the star of this afternoon Prom. There he is, swaggering through the audience, swishing his tail, nuzzling the conductor, rearing on his hind legs and – most movingly – howling in agony as he gets trapped in barbed wire. He also distracts from this Prom's many weaknesses.

At first it looked like we were getting a mimed version of the stage play, with the anthropomorphic horse joined by costumed dancers and a choir of boys in peasant garb. But the story quickly gets lost in some odd stage action. Why are there two dancers wearing sheets on ladders? Why are we being screened potted biographies of Mata Hari and Edith Cavell? And why are the house lights left on for the entire show? No wonder the children in the audience are looking confused.

Most problematic of all was the choice of music. As the poet, composer and first world war veteran Ivor Gurney observed, we've all heard of war poets, but few of us know about war composers. This attempt to assemble a canon of first world war compositions might explain why.

Some pieces are good. There's a lovely, pastoral tone poem from Frank Bridge, a Turkish folk ballad written in 1915 about Gallipoli, and a war-themed 1919 song cycle written, in German, by Paul von Klenau (later an enthusiastic Nazi). Other works, by the likes of Elgar, Holst and Ravel, have little relevance; a 1914 poem set by Henry Wood is terrible; and Gareth Malone's choral arrangements are desperately unimaginative.

One factor in the success of War Horse is John Tams' deliciously spartan folk songs, which tap into a desolate melancholy. This 90-minute show only really starts to elicit goosebumps by evoking that spirit – like when Tim Van Eyken performs an a cappella version of Only Remembered, or when Adrian Sutton knits together an overture of War Horse's themes. But, even then, you suspect that those goosebumps are just a vestigial reminder of a much better show.