MUSIC / Not showing their age: Tess Knighton on the English Concert's 20th birthday at St John's

IT IS 20 years since Trevor Pinnock founded the English Concert. That period has seen 'early music' shift from a fringe activity to a central force in late 20th-century performance practice. Two decades ago, as a student at a London music college, Pinnock had to fight to have the harpsichord taken seriously; not many years later sheer determination and conviction landed him and his orchestra an exclusive recording contract with Deutsche Grammophon. Together with Christopher Hogwood's Academy of Ancient Music, the English Concert led the period-instrument revival in this country, performing and recording many works for the first time with 'historical awareness'.

Pinnock is an instinctive musician of distinction, but the importance of his contribution to this movement went beyond bringing his own interpretative approach to the music of the 17th and 18th centuries. He strove for an orchestral sound based on a rich string tone (where others sounded edgy or even scrawny) backed by impeccable intonation (poor tuning then being the easiest target for sceptics) and technical polish. With the aid of some of the best players around - notably the former leader Simon Standage and the cellist Anthony Pleeth - he forced the musical world to sit up and take note. The group has never looked back; that it has retained all these qualities was shown by their all-Mozart anniversary concert at St John's Smith Square.

As Pinnock told the audience, Mozart is as late as the English Concert goes: 'It stretches us to the full, so that to us it seems like new music, we experience that sense of shock.' This is what they communicated in three symphonies spanning Mozart's career: Nos 26, 35 (the Haffner) and 39. The juxtaposition of these works neatly traced his development and certainly took the orchestra through its paces. But the players did not put a foot wrong: technically assured and musically alert, they brought out the exuberance of the final movement of No 39 in E flat and the fire of the Haffner's opening Allegro. Pinnock took Mozart at his word and performed the Presto of this symphony 'as fast as possible': it added to the occasion to see the first trumpet enjoying himself so much.

Pinnock has an unerring instinct for the tempo giusto. His Andantes were slower than in some other historically aware accounts of these works, but he relishes the spaciousness this affords. And in Exsultate, jubilate he was concerned less with its showpiece qualities than giving the work a rounded, humane interpretation. Barbara Bonney proved the ideal Mozart soprano, her tone pure and her technique flawless.