The Classical Opera Company / Page, St John's, Smith Square, London

How little original material does one require to make a convincing classical "reconstruction"? This concert at St John's, Smith Square, by the 36-piece, period-instrument orchestra of the Classical Opera Company, under its lively founder-conductor Ian Page, included first performances of two unknown Mozart arias completed from sketches, and a reconstructed Beethoven Largo for oboe and orchestra.

Of these, the Beethoven seemed the more questionable, for though the composer is known to have completed a three-movement Oboe Concerto in his early years, the manuscript has never been recovered. What does survive is a slightly later page of sketches, apparently for a substitute middle movement.

These comprise an opening span of melody followed by a jumble of alternatives for its continuation, to which its reconstructor, Duncan Druce, concedes that he has had to add "virtually all the harmony, subsiduary parts and the orchestral setting".

As serenely phrased by the soloist Anthony Robson, the main tune is a pleasing post-Mozartian cantilena, and at least patches of what followed sounded plausibly Beethovenian, if with the odd tonal lurch in passing. But who knows?

The Mozart arias, by contrast, seemed more solidly based. One comprised a slow rondo, "Non tardar, amato bene": part of an Act IV aria for Susanna in The Marriage of Figaro, abandoned when a change of libretto obliged Mozart to compose the familiar "Deh vieni, non tardar" instead.

In order to find suitable music for the missing fast final section, the Mozart scholar Stanley Sadie had to raid the finale of the E flat Piano Quartet, K492, written shortly after Marriage of Figaro. But at least that is pure Mozart, and the rondo itself proves well worth rescuing.

The material for the other aria, "In te spero", K440, drafted by Mozart for his wife Constanza, is evidently more exiguous: just an incomplete vocal line plus bass, implying a da-capo structure. But Sadie's filling-out, with some nicely "Mozartian" touches of wind scoring, is stylish enough to beguile. Both arias, plus "Deh vieni", were brightly delivered by the young soprano Gillian Keith, qualified only by a certain gustiness in the more florid roulades.

The concert was framed by spirited, if rather noisy, readings of Beethoven. Certainly, the wondrous processional of suave invention comprising the Larghetto of the Symphony No 2 in D major, Op 36, could have done with more genuinely hushed playing. But it was good, for once, to hear the Overture to the ballet The Creatures of Prometheus, Op 43, in its original version, running on into the quite remarkable opening storm scene.