Fervent flag-waving is usually reserved for the Last Night of the Proms, so what was going on in this Philharmonia concert? The Arena was awash with a sea of red flags, the chorus – and conductor Esa-Pekka Salonen – were decked out in T-shirts bearing portraits of Lenin or Stalin. Special Proms editions of Pravda were raised aloft, while the console of the giant organ was sheathed in idealist Soviet propaganda. In short, here was Dmitri Shostakovich thumbing his nose at the authorities in the brief Prologue to his incomplete opera Orango.

Begun in 1932, ostensibly to mark the 15th anniversary of the Bolshevik Revolution, Orango was to be a satirical science-fiction opera, telling the history of a crossbred humanoid ape. (This was just a year before King Kong first hit the silver screen.) The Prologue finds a mass state celebration under way, during which the restless crowd demands “real” entertainment: Orango, the humanoid monkey. He is duly exhibited, but attacks a woman in the crowd. A Master of Ceremonies hastily announces a performance telling the tale of Orango’s life… and there the Prologue tantalisingly ends, before the “show within a show” can even begin. A little like The Tales of Hoffmann without Olympia, Giulietta or Antonia.

Salonen and the Philharmonia attacked Gerard McBurney’s orchestration of Shostakovich’s piano score with verve, revelling in the glorious brass and percussion cacophony. A smoky saxophone crooned in the Dance of Peace, an amusing “turn” by dancer Nastya Terpsikhorova (Rosie Kay), while the members of Philharmonia Voices swayed sunflowers in time to the music. Ivan Novoselov was a game Orango, herded into his pen by banana-munching zoologist Dmitro Kolyeushko. Yuri Yevchuk (the Commissar) and Denis Beganski (Master of Ceremonies) were both a little bass-woolly, but soprano Natalia Pavlova stood out as Suzanna, the woman in the crowd who perturbs Orango.

Shostakovich’s music blends Soviet tub-thumping with raucous music hall and vicious parody into an entertaining fragment. Running at just half an hour long, Orango is a fascinating “what might have been” in the composer’s varied output, especially in Irina Brown's witty semi-staging.

The Prom certainly needed picking up after a very mixed first half. Bartók’s ballet The Miraculous Mandarin, a lurid tale of three thugs who use a prostitute to lure strangers to rob, pulsates with a violent score. Under Salonen’s direction, the violence was never overdone, lending a silky barbarity to Bartók’s score in a sensual performance. A siren bassoon wailed, trombone ejaculations throbbed, the wordless chorus moaned. If some of the jagged contours of the ballet were sandpapered away beneath Salonen’s fluid beat, its theatrical power just about registered.

There was nothing theatrical about the performance of Mozart’s Piano Concerto no. 24 in C minor from David Fray. Hunched over the keyboard, his playing was the aural equivalent of treading on eggshells. A neat, compact cadenza apart, this was soft focus, limp Mozart as disappointing as any I’ve heard all year. Fray wasn’t helped by an impotent orchestral accompaniment, glossy but glossing over a concerto that needs to glower and frown much more than it dared.