They may have been going for over a decade, but there's still something miraculous about every concert the Zehetmair Quartet play. They perform every programme entirely from memory, and the music seems to spring fully formed from the collective consciousness of the four players, led by violinist Thomas Zehetmair. But as well as the vertiginous thrill of wondering how it is possible for four people to memorise a programme like the line-up of Mozart, Bartok and Hindemith they played the Pittville Pump Room in Cheltenham, it is the interaction between the musicians that is so startling. An early Mozart string quartet in G major, K156, was catalysed by the way each gesture, like the opening of the minor key slow movement, was shaped with infinite care, each player acutely sensitive to the minute inflections of phrasing and tempo of the others.

In Mozart's later "Hunt" Quartet, the slow movement was even more affecting: Zehetmair gave an unprepossessing tune a magical musical tenderness before it was passed to cellist Ursula Smith and played with still greater gentleness and insight. But it was their performance of Bartok's epic Fifth Quartet that was the highlight of the programme. The enormous dynamic range the Zehetmairs create - from shimmering, delicate pianissimos to towering climaxes - made for a visceral experience. They revealed the astonishing textural imagination of this music, the heightened pizzicatos and bowing techniques that made the players sound like a chorus of buzzing nocturnal insects, and the unbounded rhythmic energy that Bartok conjures from the simplest of musical building blocks. The piece creates a sort of hybrid folk music, and in the Zehetmairs' performance, it was as if this rich, complex idiom was being improvised right in front of you. Not content with the challenges of Bartok and Mozart, an encore of a movement from a quartet by Hindemith was yet another demonstration of the unique alchemy between this group of players.