Czech mezzo Magdalena Kozená was the soloist in this Barbican appearance by the Canadian chamber orchestra Les Violons du Roy under its founder and music director Bernard Labadie. Always clear and direct in her musical instincts, she nevertheless gave a vocally uneven performance, though one that improved steadily as she went on.

Her contribution to the first half, Haydn's substantial cantata Arianna a Naxos – heard in an anonymous orchestration by one of his contemporaries – was marred by physical and vocal tensions that undermined the expressive potential of her interpretative gestures. Here as elsewhere in the programme, security lay in the middle of her voice, with the top approached with more care than confidence and the bottom register at times barely cutting through. If she emerged from the experience largely unscathed, there were some uncomfortable moments.

In any event, Kozená's three arias from Mozart's La Clemenza di Tito after the interval saw her on more relaxed form, with most of the technical hurdles – the triplet runs in Parto, Parto, for instance – neatly surmounted. It's a repertoire that suits her, even if some low passages in Non Più di Fiori descended into the guttural.

The 27-strong orchestra, meanwhile, outshone her in beauty of tone and brilliance of attack, Labadie drawing spry and energised playing from his musicians in symphonies by Mozart and Haydn. He captured the sophistication and urbanity of Mozart's Symphony No 33, while displaying the wit and sheer ingenuity of Haydn's Symphony No 85, La Reine, with style and relish. Elsewhere, he managed the accompaniments to Kozená's arias sensitively, including in her disarming encore, Voi Che Sapete from Figaro, while Wolfgang Meyer delivered his basset-horn and clarinet obbligatos with impeccable virtuosity.