MUSIC / A voyage round the horn: Meredith Oakes hears the Ensemble Sonnerie play sonatas by Fontana at St James's, Piccadilly

THEY called the concert 'Birth of the Sonata', and the sonata seems to have been born like the blues, when they took a horn, etc. The composer featured at St James's, Piccadilly, on Friday night was Giovanni Battista Fontana, a 17th-century Italian who wrote his sonatas for violin or cornetto interchangeably.

Although Fontana was a virtuoso violinist it was the cornetto, that mellifluous, tricky, drinking-horn-shaped ancestor to - what? The modern trumpet? The saxophone? - which was the instrumental star of the century.

The ambling language of these old sonatas, with their fanfares and fiery runs, their stately processional tempos and their antiphonal calls from one soloist to another, was cornetto- centred. Fontana made the first steps towards developing a nimbler string rhetoric which with Vivaldi was to become all-powerful: but that was much later.

Monica Huggett played one of the two solo violins and inspiringly directed her Ensemble Sonnerie, which embraced two cornettists as well as continuo and bass instruments. The keyboards and chitarrone omnipresent in this context were never solo, always continuo, playing strong rhythms and bass lines in patterned chord sequences. The sound was Monteverdi: major and minor, double- and triple-time deliciously intertwined. Violins and cornetti languished, declaimed and embellished like opera singers, though they also embarked on fast, showy divison passages that were essentially instrumental.

Numbers varied - anything from one soloist to six. The two biggest ensembles were for works by Fontana's contemporaries Buonamente - a fairly rudimentary mosaic of fanfares and runs for two violins versus two cornetti, Sonata Prima a 4 - and Legrenzi - Sonata 'La Buscha' a 6 - more thematically varied and more briskly fugal. Here the four high soloists were joined by the violone and by the 17th-century bassoon, the curtal, a buzzy, buffo instrument lacking the romantic legato mournfulness of its modern descendant. In various Fontana pieces, as in the Legrenzi, it was striking to see how fully characterised the curtal was, set apart from its colleagues in isolated passages that sounded like ponderous folk-jigs.

The many Fontana pieces came from a posthumous collection of his sonatas published in 1641; the Ensemble Sonnerie will record them for Das Alte Werk. This is Arcadian music where every sighing cadence and every rapid flight of elaboration breathes pleasure, if in a predictable way.

The biggish Sonata Quinta for solo violin, and the moody Sonata 11 for two cornetti, were two that seemed particularly well worked through, with fertile, contrasted ideas explored at length. Maybe Ensemble Sonnerie should bring out a pop single: the durations are about right.

The playing was seductive and unobtrusively fastidious. Everything fell into place with such throw-away grace that the excellent cueing, and the swaying unanimity of style, seemed spontaneous. Even in these days when good cornetto playing is not as freakish as it was 20 years ago, Bruce Dickey and Doron David Sherwin achieved breathtaking agility at what must be a virtuoso peak within the repertoire.

Lufthansa Festival of Baroque Music continues tonight. Musica Antiqua Koln presents music by 'Mozart's Friends' at 7.30pm at St James's Piccadilly (071-434 4003).