Music Malcolm Arnold 75th Birthday Concert Barbican Centre, London

In the almost unbelievably terse programme note to his Fifth Symphony, Malcolm Arnold writes: "It will be noted that, in the second movement, the composer is unable to distinguish between sentiment and sentimentality." Another, older note provides a different slant: "In times of great emotion we speak in emotional cliches."

Somewhere between these two statements is the key to Malcolm Arnold. The slow movement of the Fifth Symphony begins with one of Arnold's most cinema-weepy tunes. At the end of the symphony it returns for a fulsome, if not quite complete Hollywood apotheosis. Of course it's a cliche, but in context it can be disturbingly touching. Is this a testimony to an exceptionally intense, raw emotion? The rest of the symphony ranges in mood from uneasiness, through keen-edged sarcasm, to desperation. Inevitably one thinks of Mahler, perhaps also the bleak, dislocated circus music of Shostakovich's Fourth Symphony, or the gallows humour of Nielsen's Sixth.

A lot depends on the performance, but Richard Hickox and the London Symphony Orchestra gave the most convincing account of the work I've ever heard in Sunday's Malcolm Arnold 75th Birthday Concert. Interestingly, Hickox didn't try to point the irony or sweeten the schmaltz in that deplorable slow movement tune; he simply took it at face value - and the impact was enhanced. Likewise the shock "break-up" of the theme at the end: the gesture is theatrical enough in itself.

But what is sentimentality, and what differentiates it from sentiment? CG Jung defined the former as a superstructure overlaying brutality. Works like the Fifth Symphony suggest that Arnold would agree: in the end, the brutal seems to have the upper hand. And in a work like the later Fantasy on a Theme of John Field for piano and orchestra, brutality dominates, the superstructure seems alarmingly flimsy. Soloist John Lill again took the music very much at face value, and a very black, pessimistic statement emerged.

And yet, before the Fantasy, there was the first set of English Dances - the first dance sentimental as hell, but also fresh, touching, melodically irresistible. Who writes tunes like Malcolm Arnold today? Not Andrew Lloyd Webber and, it has to be said, not George Lloyd. And before the Dances came the Tam O'Shanter overture: shamelessly illustrative programme music, full of "och aye the noo!" fake Scots tunes and music-hall theatricality - and quite wonderful. It seems many would-be sophisticates still find it easy to ignore Arnold's compositional skill, but the technique is masterful - one could learn as much about orchestration from his scores as from any other English composer.

But it was the Fifth Symphony that left the deepest imprint. Like Mahler, Arnold plainly believes that a symphony should "embrace the world". The result is music that both mirrors the age of its birth and looks forward darkly. The Fifth appeared in 1961, the beginning of a period of almost insane optimism. With hindsight, the music seems at once to acknowledge all that and to see through it. When the current categories of artistic "progressivism" and "conservatism" pass away (as they almost always do), Malcolm Arnold's Fifth Symphony may come to be seen as one of the deeper musical commentaries on its time.