Proms CITY OF BIRMINGHAM SYMPHONY ORCHESTRA / SIR SIMON RATTLE Royal Albert Hall, London / BBC Radio 3

On the other side of the interval, Mahler's Sixth loomed. And perhaps it was that prospect which seemed to sharpen and intensify the little chromatic warnings given out in Mozart's Piano Concerto No 18 in B flat. Imogen Cooper and Sir Simon Rattle went in deep: deeper and darker with each successive deception. All is not what it seems, here. The greater the charm, the deeper the deception. Or was that the proximity of Mahler playing tricks again? Cooper and Rattle are a finely tuned partnership: the very idea of a complacent phrase is inconceivable. All of which served to heighten and reinforce the intrigue. Who could really know where the slow movement's strangely obtuse variations were about to lead us.

Mahler's Sixth Symphony. And so the terrible ordeal began again. Nowhere in the history of Western music does man strive so high, only to be brought so low. I like Donald Mitchell's assertion that, after Strauss's Death and Transfiguration, Mahler's finale might be entitled Death and Extinction. Rattle has been this way before - many times. He's lived and died this symphony for nearly two decades now. And perhaps he has journeyed far enough in time and space from Mahler's own superstitions to be able to reinstate the third and final hammer blow.

To hear it in situ, sounding disturbingly as though another crack had just appeared in our universe, is to understand why Mahler deleted it. Rattle's percussionist turned it into a threefold ritual: a black cloth ceremoniously laid down on the sound-box, the hammer raised and swung like a vengeful Thor. Mahler's acute sense of the grotesque was duly satisfied. And, in response, the CBSO brass resoundingly laid bare his blackest counterpoint.

It was, in a word, stupendous. There isn't a note of text here that Rattle hasn't digested, re-evaluated, understood - the musical reasons, revelations, revolutions. The evolution of Alma's theme - the first movement's second subject, sung in the strings, shouted in unison clarinets (bells raised high over the music-stands), embellished in descanting horns (the entire first movement development is about the aspirations of this theme) - was never clearer or more poignant. Its final bid for the ascendancy in that great climactic chord of the coda (like a giant suspension heard in slow motion) was truly all-consuming. But then, everywhere harmonies were newly revealed, textures re-opened, colours re-primed (what galaxy does that opening paragraph of the finale come from?).

There remains one contentious issue: the ordering of the middle movements. Evidence exists to suggest that Mahler did revert to his original ordering - Scherzo-Andante - before his death. But Rattle remains unshakeable in his belief that Andante-Scherzo is the right musical sequence and, as ever, his arguments (centred on harmonic structure) are compelling. I happen not to agree. To take that terrible plunge from the first movement's triumphant A major back into the minor, to have to surrender once more to a distorted mirror-image of the first movement's dogged march, is to experience one of the most daring and uncompromising instances of sustained terrorism in all music. Could it be that Mahler simply lost his nerve prior to the premiere? I must say that, as Rattle's violins eased their way into the consoling opening measures of the Andante, I felt a little as though - for once, and once only - we'd been let off the hook.