What strange bedfellows Schubert and Wagner are when placed side by side in a concert programme! They are poles apart, both as personalities and in their technical strengths – as Daniel Harding and the London Symphony Orchestra showed us at the Barbican when they performed Schubert’s “Unfinished” Symphony alongside the second act of Wagner’s Tristan und Isolde. It is stranger still to think that Schubert was a mere 16 years older than the 200-year-old Wagner. This juxtaposition of mature works by these composers demonstrated clearly just how fast compositional styles developed in the early part of the 19th century.

The chaste purity of the once ubiquitous “Unfinished” Symphony was brought to life in a finely judged performance here. The obscenely melodically rich first movement had just the right balance of poise and angst. The dramatic development section (surely one of the greatest moments in all music) was brought off with taste and passion, so that the dissolve into the first subject’s shimmering strings was a divine moment. Likewise the second movement kept the mood-swings in balance and the “walking pace” tempo was spot on. Though, as it should be, this complex journey was no walk in the park.

After the interval we were thrust into the equally rarefied world of Wagner’s Tristan und Isolde. After the Schubert it felt, at first, almost tawdry – elevated passion with too much flesh on the bone. Overblown orchestral forces and overripe singers blasting out for all they are worth – but once this initial shock subsided it was possible to settle down and just be swept along by what must be one of the most extraordinary love duets in all opera.

However high-minded Wagner’s own libretto is, the music is pure sex. The passage when the lover’s meet alone for the first time, is hysterical and disturbing and after that subsides, the gradual build up towards the coitus interruptus is spellbinding and sensual. By then, all thoughts of Schubert’s lonely and anxious world were long gone.

And this was a performance to confirm the true musical genius of the piece. Harding and the LSO seemed to relish the score from first to last note. One felt that both orchestra and singers could let themselves go, knowing that they didn’t have to pace themselves over three acts.

The basic tempo set by Harding was fast, always a good thing in this act, as it helps build the necessary intensity. With the orchestra on stage and not in a pit, the benefits of being able to concentrate and appreciate the complexity and richness of the orchestration were in evidence. This can lead to the singers being drowned out, but in this performance that rarely happened. This was due to a number of factors – firstly, Wagner’s scoring is surprisingly careful, allowing the voices to dominate the sound when they need to. Secondly, Daniel Harding worked hard to grade the orchestral sound where there was most strain on the singers. And then, the group of singers cast for the performance all had exceptionally large voices.

And it was indeed a fine vocal performance all round, notably from Iréne Theorin, whose relaxed and arrogant portrayal of Isolde seemed to hit the nail on the head, capturing the essence of the flighty youthful beauty. She encompassed the huge dynamic range of the part with ease, managing to control the passionate outbursts with a thrilling displays of vocal power and producing some thrilling top notes. She was also able to scale her voice down to be meltingly seductive as required in the quiet, rapt moments. Her Tristan, Peter Seiffert, was equally responsive and was suitably heroic in tone. But the other really outstanding performance, alongside Theorin, was that of Matti Salminen as the poor, cuckolded King Marke, whose difficult monologue was given a poignancy and musical point that I haven’t heard so well done – so often a damp squib after the fervour of the lovers’ encounter. Christianne Stotijn did her very best with the thankless role of Brangäne, with beautiful tone in her incantations while on watch. Mark Stone dispatched the small roles of Merlot and Kurwenal with aplomb.

This was a performance which focused on the music, with the orchestral part given a performance that it would be hard to match by the LSO on top form. Harding seemed to have a natural understanding of the Wagnerian pulse – it would be good now to hear him conduct the whole work in a stage performance with this cast.