“Q: Why can you often find viola players standing outside your front door? A: They can't find the right key and never know when to come in!” Viola players are the butt of a whole genre of jokes, but Berlioz’s Harold in Italy – sometimes dubbed “the world’s longest viola joke” – is in a category all of its own, a lengthy non-concerto based on Byron’s Childe Harold in which the hero is often silent. How wonderful, then, that Antoine Tamestit had the last laugh in a brilliant all-Berlioz Prom by the period instrument Orchestre Révolutionnaire et Romantique under Sir John Eliot Gardiner.

Harold was composed at the request of Niccolò Paganini, who had acquired a Stradivarius viola and wanted Berlioz to write him a concerto to show it off. Yet when Paganini was shown the sketches for the first movement, containing many tacet bars and free of technical virtuosity, he was not amused and flounced off in a huff. Berlioz completed it as a programmatic symphony “with viola obbligato”. It wasn’t until four years after the première that Paganini finally heard Harold performed in concert and was so moved that he dragged Berlioz to the stage and knelt to kiss his hand.

Like Caspar David Friedrich’s Wanderer above the Mist, Harold is a bystander, a melancholy observer. Antoine Tamestit played this outsider perfectly. When Gardiner ushered in the brooding introduction, Tamestit was nowhere to be seen. He eventually emerged through Door G, tentatively descending the steps as if a latecomer looking to locate his seat. Demonstrative in his movements, Tamestit was physically drawn into the performance, clambering behind the tangy horns like a mountain goat in the March of the Pilgrims, strumming his viola during the wheezing woodwinds’ rustic serenade, before disappearing altogether – literally – during the brigands’ noisy orgy. (Seriously, was there a trapdoor?) Berlioz then forgets all about his soloist for page after page until two violinists and a cellist, scaling the platform to perform a distant trio, were spotted by Tamestit, who joined them. It’s as if Berlioz has the viola shrinking away from the limelight, finding its natural habitat at the heart of a string quartet.

Tamestit acted his role perfectly, with Gallic wit, his lean tone ranging from deliberately scuffed lower notes to a warm caramel centre and a slightly astringent top and wiry sul ponticello effects. This was a vivid performance that made me seriously re-evaluate Harold as a work. That it overshadowed a wonderful first half of the concert was astonishing in itself. Lord Byron was also present in the opening number, a swashbuckling, salty Corsaire overture. Gardiner had his players standing (as they did in the Brigands’ Orgy), generating terrific energy, four bassoonists bobbing, ophicleide grunting greedily.

Mezzo-soprano Joyce DiDonato then died – twice – in a second “serpent and fire” pairing of the season following Anna Prohaska’s engaging Late Night Prom focusing on Dido and Cleopatra. La Mort de Cléopâtre is a dramatic scena, based on Shakespeare, while Didon’s suicide on the pyre closes Berlioz’s epic opera Les Troyens. DiDonato had to force her chest register once or twice in Cléopâtre, but her ascents into soprano territory were seamless, her dramatic instincts fierce, flinching at the asp bite, slithering double basses chilling the blood. Her Didon, which so impressed me in Strasbourg, was immensely moving once again, particularly in her dying moments where, recalling the theme of the love duet with Énée, DiDonato scaled back her voice to the merest thread.

The Royal Hunt and Storm episode from Troyens reached an appropriately thunderous climax, peripatetic saxhorns rasping either side of the stage, gunshot timps at the rear and – magically – the brief choral contributions coming from the female ORR string players. As in the rest of this knockout Prom, Gardiner thoroughly embraced Berlioz’s theatrical eccentricity and the results were riveting. Tamestit and DiDonato then joined forces for "The Ballad of the King of Thule" from The Damnation of Faust as a silky encore.