Julia Fischer, Martin Helmchen, Queen Elizabeth Hall

An entire evening of Schumann for two would usually cue singer and piano. Not that the majority of Lieder specialists, blessed as naughty Anna Russell once saw it "with tremendous artistry but no voice", could hold the spell for that long. Julia Fischer is one of the half-dozen violinists in the world with the greatest artistry, a golden "voice" and a habit of choosing partners like Martin Helmchen, very much on her level. The only trouble is that Schumann songs can capture a world in 90 minutes, while the three lateish sonatas run a more limited if quirky gamut.

Poor Schumann was already in decline by the time he wrote the first two in 1851, though you wouldn't necessarily know it; the white-heat melodic and rhythmic inspiration of the earlier works has given way to a singular thematic mapping-out and rather more broken piano figurations. But the chief pleasure for any duo must be the way he shares out themes and ideas between violin and piano, and that fusion was vividly embodied by Fischer and Helmchen right from the start of the A-minor Sonata. The violin line melts into the piano's, questions it and receives wayward answers - and all this in a structure these two made seem unerring and compact, a canny fusion of Classical and Romantic.

The finales are less convincing, for all the rippling dialogues of the 1851 sonatas. The one in the A-minor Sonata of two years later rings hollow, which may have been partly because the nimble, subtle Helmchen doesn't really do rampaging-jubilant, though his violinist knows no limits and rightly played the hero in the double-stopping of the grand D-minor work. For all that, it's clearly in the aphoristic inner mood-musics that the soul of the composer and his flickering love for Clara are embodied. It's frankly a relief when Op 121 lays off the rodomontade, and there's nothing more arresting in late Schumann than the spare, simple variations of its slow movement. If its theme sounds "chorale like" to the programme annotator - who at least came close to essaying what we heard here, if hardly anywhere else in slabs of under-flavoured Impressionism - that's because it is a chorale ("Gelobet seist du, Jesu Christi"), though uniquely personlised from pizzicato through to skeletal song.

Even stranger is the intermezzo of the last, outsider sonata, with typically striking piano figurations poetically rendered by Helmchen and a line above it voiced with typical poise by Fischer as a kind of disembodied descant, as if the real melody goes unheard in between. The duo were right to present it as an encore, restoring to us the real Schumann after the bluster of a third unconvincing finale. The abiding impression remains, though, of a subtle but rather subdued evening, not much assisted by the unwelcoming starkness of the QEH and the waft of burnt sausages invading the hall from a Liberty humanitarian awards function in the foyers.

• Find the Fischer-Helmchen partnership's Schubert recordings for Pentatone on Amazon
• See what's on at the Southbank Centre this season