La Boh&egrave;me, Cock Tavern Theatre, Kilburn, London<br/>Jansons / Royal Concertgebouw Orchestra, Barbican, London

You'd have to drink the Cock Tavern dry to mistake Kilburn High Road for the Quartier Latin, and downstairs from the pub's tiny 40-seat theatre, where Opera Up Close unveiled its production of La Bohème last week, that was exactly what some of the locals were doing.

Sandwiched between the organic bakeries and gastropubs of Queen's Park and the portered mansion blocks of Maida Vale (both of which are name-checked in Robin Norton-Hale's updated English libretto), Kilburn is an area of pound shops and betting shops, casual labour and cheap sublets. A nice verismo touch, this – it also has the highest rate of tuberculosis infection in Greater London.

With a cast in their early twenties, a battered sofa, laptop and laundry airer for props, an out-of-tune piano for an orchestra and several dozen bemused drinkers as the supernumeraries for Act II, when the production moves to the bar, Norton-Hale's Bohème is certainly plucky. All involved are working for a putative share in the profits, bringing a glow of youthful optimism to Puccini's bitter little tragedy. Though the singers have coltish voices, still cautious in the transition from soft to loud, low to high, the characterisation is immediate and authentic, the confusion and selfishness of first love powerfully conveyed in the contemporary setting.

No need to change the job titles for modern Kilburn. Rodolfo (Christopher Diffey) is still a writer, insecure and petulant, Marcello (Richard Latham) an unsuccessful artist but a pragmatist in love. Too big a personality for a bedsit, Schaunard (Brett Brown) is an aspiring musician, gentle Colline (Alistair Sutherland) a baby-faced philosophy graduate, his moustache still soft and thin. Their landlord, Benoît (Ian Wilson-Pope), isn't long in London himself, perhaps from Bosnia or Serbia, while Mimi (Charmian Bedford) is another migrant, working as a cleaner and sewing floral brooches by night. As to poor, flustered Alcindoro (Tom Murphy), his employment is immaterial. He's just there to pick up the bill when Clare Presland's tough, tart, true-to-you-in-my-fashion Musetta waltzes out of the bar with her on-off painter boyfriend to whoops of appreciation from the locals.

So far, so cute, with plenty of extramusical charm. But as the singers adjust their voices to the muggy acoustics of the upstairs room, grow into their roles and the story turns sour, you notice a deeper layer to Norton-Hale's work: the careless arrogance behind that skit with Benoît, the way in which Rodolfo treats Mimi as an idiot-savant, tickled by her naive delight in her fabric flowers. To the four young men, all from middle-class families, living hand to mouth is a picaresque fantasy.

To Musetta and Mimi, poverty is real, exhausting, deadly. Strike One comes in the Act III quartet, when Marcello calls Musetta a "chav" and she calls him a "posh git". Strike Two is when consumption claims its victim, still believable today, when there is every chance that an economic migrant might be too scared or weak to fight her way through the queues at the local clinic, too afraid of losing employment to take time off.

This is not opera at its grandest or most beautifully sung. But it is honest, perceptive, unpretentious and intelligent and a breath of fresh, beer-and-crisp-scented air for a work that thrives on reinvention, 103 years after its premiere.

Cut off from its Beethovenian roots, Mariss Jansons's interpretation of Brahms's Fourth Symphony with the Royal Concertgebouw Orchestra was dreamlike, disorienting, episodic. With each cross-rhythm lifted out of context and held up to the light, each transition bathed in a sheen of sumptuous sound, the lieder-like lilt of the Allegro ma non troppo slowed to a balmy andante, the Andante Moderato to something approaching adagio. Neither Jansons nor the Concertgebouw has a reputation for Brahms, and on this showing, coupled with a gratuitously fast rendition of the Overture to The Bartered Bride and Martinu's scowling Double Concerto for two string orchestras, piano and timpani, I rather wish they hadn't tried to address that deficit. Yet having lost the plot of one of the most clearly wrought symphonic narratives, they navigated Mahler's Resurrection Symphony with radiant, revelatory ease.

This was, like Douglas Boyd and the Chamber Orchestra of Europe's Symphonie Fantastique, Franz Brüggen and the Orchestra of the 18th Century's Schubert Unfinished and Antonio Pappano and Nina Stemme's Liebestod, a performance that I cannot imagine I will hear bettered: profoundly felt and poised, clear-sighted, apprehensive rather than neurotic in the opening movement, vigorous yet unhurried, with the off-stage brass opening up a vista of infinite redemption, sorrow, beauty and ecstatic stillness. I was not the only person weeping during Bernarda Fink's rapt "Urlicht". Half of the London Symphony Chorus was in tears, though in wonderful voice for "Auferstehen".

That Jansons and the Concertgebouw can produce something so monumental yet almost weightless, so vividly detailed yet organic a narrative, is almost incredible. That this marked the launch of a series of Barbican residencies is the best possible news for the Mahler anniversaries in 2010 and 2011.

'La Bohème' (0844 477 1000), to 23 Jan