MUSIC / Five into four will go: Adrian Jack on quintets from the Alberni Quartet's quintet recital and friends

The most prolific composer of string quintets was Boccherini - the famous Minuet comes from one of them - and, being a cellist, he supplemented the string quartet with a second cello. Mozart had a particular liking for the viola, so his string quintets add, instead, a second viola. Last Saturday evening that put the long-established Alberni Quartet in the position of finding two extra players, Ivo-Jan van der Werff and Philip Dukes, since their regular violist, Roger Best, is currently indisposed.

For their programme at the Wigmore Hall they had chosen three of Mozart's six string quintets, including the earliest, in B flat major, and the best-known, in G minor. Best-known, because of the irresistible poignancy of its first movement and the tender vulnerability of its Adagio, where one of the most unforgettable moments is a mere accompaniment in which the second viola throbs like a raw nerve on its lowest string. Never mind all those viola jokes, Mozart recognised the instrument's unique shadowy richness without over-working it.

Viola-players are often wooden and inhibited in the way they play, but these two were fluent and expressive. In fact, it was the Alberni's cellist who tended to be faint- hearted, making his little rising arpeggios in the slow introduction to the D major Quintet sound like rather uncertain excuses and, no doubt by prior agreement with the other players, cutting rests.

Unlike keyboard-players, string-players have no difficulty in shaping an eloquent phrase, since it's in the nature of their instruments to sing. But they do tend to be insecure about rhythm and tempo. Given half a chance, the music will slip forward out of their control, like a slippery eel; and, opening this concert, both the first two movements of the B flat Quintet had that tendency.

More subtly, the Alberni's rhythm sometimes lacked tension, a sense of being sprung, so that the Minuet of the G minor Quintet, for instance, seemed to walk rather than dance. Which also had something to do with a lack of precise contrast between loud and soft - as if the players thought that softness was merely a negative quality.

But the first thing people notice about string-players is intonation, and there wasn't much cause for complaint there, except in the problematic finale of the D major Quintet, with its air of insouciant humour, which sounds like nothing so much as Rossini, who wrote a set of witty string sonatas when he was still a boy, barely more than a decade after Mozart's death. If I say I'm not quite sure what the first violinist played, it's not entirely to find fault with his intonation. The murderous little descending chromatic patterns that Mozart wrote were apparently altered by someone else and further simplified, possibly by his publisher, so there remains an editorial choice of what to play. Such details are genuinely hard to identify as they skitter by in a performance; none the less, the effect was enchantingly queasy.