The poetry of Arthur Rimbaud was, to borrow a phrase from the 1960s, "way out there." His 1870s collection Les Illuminations inspired Benjamin Britten, who set a selection of the poems to music: the words are surreal, bizarre, sometimes erotic, always onomatopoeic and packed with interesting sounds. Britten's music brilliantly builds upon and amplifies Rimbaud's rhythms, phrasing and imagery, producing a potent and fascinating. It's an awful pun, but it's fair to say that Britten's music illuminates Rimbaud's poetry.

It's a brave work for an English tenor to take on in front of a largely French-speaking audience in the art deco splendour of the Palais des Beaux-Arts in Brussels, but Ian Bostridge clearly revelled in the task. Bostridge slid his voice sinuously around the complex phrases, voicing the rugged consonants with gusto. He was elegiac in the brief romantic passages and portentous in the impressive refrain J'ai seul la clef de cette parade sauvage ("only I have the key to this wild parade") which punctuates the dreamscapes. The audience were in raptures.

Roger Norrington and the Mahler Chamber Orchestra supported him ably. Norrington has an unusual conducting style: I imagine he feels that the real work has been done in rehearsal and so does very little on the podium: there's no baton, and just a gentle nudge here and there while the players get on with the job. It worked just fine in the Britten, giving a shining demonstration of the composer's cleverness in splitting and recombining the different instruments to produce a wide variety of tones and textures.

However, the evening did not get off to a good start with a set of passages from Mozart's Idomeneo, which demand a tenor with a completely different voice type. Bostridge may have been ideal as the tortured aesthete dreaming scenes of surpassing weirdness, but I didn't find him at all believable as the heroic but doomed king of Crete. Norrington and the orchestra didn't help: their playing was accurate and the pace energetic, but there was little dynamic range, little accenting and the sound balance was overly biased to the lower ranges. Bostridge was overpowered to the point of being barely audible, and the wind phrases that lend so much subtlety to Mozart's music had little room to breathe.

When the full Mahler Chamber Orchestra returned after the Britten to play Schubert's Unfinished Symphony, such was the difference that I could hardly believe that I was listening to the same orchestra and conductor. Before the concert, one of the orchestra members told me that the great thing about working with Norrington is that even in a familiar piece like the Schubert, he will find a new and interesting way of interpreting the music. Norrington's magic worked for me: I loved the space which was given to each of the instruments as Schubert's melodies were woven together, and I loved the constant motion between different levels of intensity. I don't consider the Unfinished Symphony to be a piece that I know particularly well, and yet I was struck by how much of the music was completely familiar. I put it down to Schubert's gift for melody: there are so many themes in the work and they're so memorable that even if I've only heard them a few times in the past, the memories come flooding back when you hear the whole work.

An evening of two parts, therefore: I was unimpressed by the Idomeneo but completely bowled over by the Britten and thoroughly lifted by the Schubert. The only problem was that the Unfinished Symphony is, er, unfinished. At the end of the concert, I was just ready for a scherzo...