Preview: The Belcea Quartet, Wigmore Hall, London

A string quartet by Schubert or Beethoven is a distilled symphony: the musical lines are sharpened, the feeling refined – chamber players are the aristocrats of the classical world. When the Belcea Quartet play these composers at the Wigmore tomorrow night, that truth should be nicely exemplified. Established while still at the Royal College of Music in 1994, this youthful group – whose first-class recording of Bartok's String Quartets is just out on EMI – have become brand-leaders for their generation.

Their most recent recruit is the French cellist Antoine Lederlin, who moved in two years ago to fill a vacancy, but only after the most rigorous vetting. "I didn't know them, but they knew about me, and one day, out of the blue, they called and asked if I would be interested in joining – but they were also going to audition others. I spent three days with them in London, working on three quartets, and then there was silence. Two months later they rang again and invited me to tour with them, and at the end said that I was welcome to join, because it had worked between us."

But they're still learning: when I catch Lederlin, he's visibly wrung out after a gruelling four-hour tutorial the quartet has just had with pianist Alfred Brendel. "He has no interest in the practical, technical problems we string-players have to grapple with – all that concerns him is the musical effect he wants!" But Brendel doesn't want payment: he does it because he believes in their potential.

On the other hand, the Belcea have just been installed as quartet-in-residence at the Guildhall, where they are passing on what they have learnt. They find their situation there frustrating, since the chamber course is optional and students come and go. "They don't realise that quartets are, ironically, more demanding than solo work, and quite different from trios," says Lederlin, who works in all three modes. "In a quartet there's nowhere to hide, so the communal precision must be perfect. A lot of our work is simply to maintain that."