Classical: Austro-Hungarian Orchestra; Queen Elizabeth Hall, London

Convention has it that Haydn is bad box-office and so are Mondays. But on two successive Mondays - last week at the Barbican and this week at the Royal Festival Hall - large, appreciative audiences greeted two orchestras, both from similar parts of the world, one in a concert entirely of Haydn, the other largely Haydn, where "conventional" was hardly a suitable description.

Visits by Nikolaus Harnoncourt are less rare nowadays, but visits by Harnoncourt with his orchestra, the legendary Vienna Concentus Musicus, are much rarer. His Barbican performance kicked off a new series under the inauspicious title of "Group Dynamics" in which artists are shared between concert halls in London and Berlin.

Concentus Musicus is a "grand old group" in historical performance terms. Not until they had rehearsed for four years did they venture to give their first performance, and that was back in 1957. Nowadays, judging from the age of most of the performers, few, if any, can have experienced that intensive study. In two symphonies (Nos 52 and 31), Harnoncourt allowed the valveless horns to have their heads, so that they dominated the texture to startling effect while showing that as instruments they're as hard to tame as the animals they're supposed to hunt out. It was not until the arrival of that impeccably musical soprano, Barbara Bonney, that much heart came into the proceedings. In two arias from Il mondo della luna and L'infedelta delusa, and the dramatic Scena di Berenice, Bonney was in radiant voice, blessing her notes with largely forbidden vibrato.

Adam Fischer's Austro-Hungarian Haydn Orchestra is, compared with Concentus Musicus, very young indeed, founded only in 1987, although the members generally look older. The orchestra, normally no more than 45 (the number Haydn had available to him), is drawn from soloists, chamber musicians and members of leading orchestras in both Austria and Hungary.

If Harnoncourt is head, Fischer is heart. The playing is informed by a grace and lightness far removed from schlag und schmaltz. Within small- scale interpretations, Fischer attends to the minutest detail with loving care that is exquisitely musical. In Haydn's overture to La fedelta premiata and the Farewell Symphony (No 45), he drove a dramatic punch and was rewarded by scintillating ensemble playing. For Mahler's Fourth Symphony, the orchestra virtually tripled on stage, although this was still small-scale Mahler. Fischer brought lightness, the big climaxes good-willed rather than menacing. His "attacca" into the fourth movement brought a palpable gasp from the audience as tension from the emotional rack of the third movement was released by the music rather than by the arrival on stage of an over-decked soprano. Patricia Rozario sang simply and sensitively; a delight met by the most quietly attentive audience I've witnessed for some time - unconventional indeed.