Les Arts Florissants, Union Chapel, Islington

Of all the ensembles celebrating an anniversary this year, William Christie and Les Arts Florissants - now toasting three decades of ground-breaking experiment - have most reason to congratulate themselves. Thirty years ago, when they embarked on their still-ongoing project to bring French Baroque music into focus, they were tilling virgin territory; if it’s now all flowers and fruit trees, that’s largely thanks to them. Moving on from Charpentier (who gave them their name) to Purcell, Handel, and Haydn, they have continued their quest: hats off to the Barbican, which - in collaboration with the Salle Pleyel and the Cite de la Musique - has commissioned their lap of honour this autumn.

To catch them in Islington's Union Chapel - usually a venue for rougher kinds of music - was to find them in peak form: this performance of Monteverdi's sixth book of madrigals honoured the sublimity of the music as I have never heard it honoured before. Six singers, backed by harp, archlute, theorbo, and harpsichord, seemed a very small army to take on this massive stone space, but they effortlessly filled it with gossamer flights and astonishingly dark bass sonorities.

Monteverdi's Book Six was composed when he was going through a terrible vale of sadness, with his young wife dying, and his star soprano Caterina Martinelli - virtually his adoptive daughter - being carried off too. His now-lost opera 'L'Arianna' had been written for Martinelli: all that remains to us is a solo version, and a five-voice madrigal arrangement, which is what the singers began with here. It really did sound as though the anguished opening phrase 'Lasciatemi morire' - 'Let me die' - was the composer speaking: as the four-part piece ran its course from despair to denial to rage to numbed acceptance, we were swept irresistibly along too.

As the multiplicity of recordings now proves, there are a thousand ways of getting Monteverdi wrong - or just not quite right, which is almost as bad. Some groups are prissily over-precise, some are too dry, some are too emotionally steamy: since performance styles are necessarily a matter of conjecture, informed artistic judgment is crucial. Spare and intimately conversational, yet generating huge power, Les Arts Florissants hit the mark unerringly. They can breathe and sigh as one instrument, and they can fragment into a medley of angrily contending voices; their line is pure, with only the merest hint of vibrato; with them, meaning is all. Each of the singers had their own unique sound, but two or three stood out - notably Sean Clayton, possessor of one of the most delicately expressive tenor voices I have ever heard; tenor Paul Agnew was the ideal singer-director.

Suiting the madrigals' different demands, they arranged themselves in constantly changing permutations, and in a variety of formations - a line, a circle, a horseshoe. When it came to 'Sestina: Lament of a lover at the Tomb of his Beloved', they dispensed with instrumental backing and sat in a semi-circle with their knees almost touching; the pathos of their delivery was all the more persuasive. When the mood lightened, with the music evoking sunlit valleys, flocks of birds, and the wind in trees, everything was conjured up with the most exquisite artistry. This concert simply got better, as the palpable concentration of the audience fuelled a similar intensity of focus on stage. And one realised anew how modern this music is, with its daring chromaticism, and its long-suspended muscular dissonances.

Those wanting to hear Les Arts Florissants singing Monteverdi on disc can do so thanks to the 30-Cd set which Warners have just released. The final concert in this series is on 26 November, when William Christie conducts the ensemble in 'Grands Motets' in the Barbican Hall.