Birtwistle Games/Philharmonia, Royal Festival Hall, London

The second of the Philharmonia Orchestra's two Birtwistle Games concerts was a curiously fissile affair, spun off from the programme's main work, The Shadow of Night, and its origins in Dowland. With splendid disregard for the likely predilections of Birtwistle's own following (a good number of whom stayed moodily in the bar), the first half was entirely Dowland, performed by the New London Consort under Philip Pickett. After the interval, Christoph von Dohnányi conducted The Shadow of Night together with its own recent shadow, Night's Black Bird, and in between Joanne Lunn sang the source song, "In darkness let me dwell", popping up with coy determination like Sweet Polly Oliver in the middle of Birtwistle's huge, imperious orchestral army.

This sense of origins in Birtwistle - his love of pictorial imagery and distant musical ancestry - certainly eases the thorny pathway into his more forbidding scores. But the music itself proceeds on its lonely, utterly uncompromising way, marvellous to its admirers, inexplicable to everyone else. The Shadow of Night is a densely intricate 30-minute work that hacks its way through tangled polyphonies and the haunts of strange nocturnal instruments. It can be either preceded or followed, the composer says, by Night's Black Bird, a short cut through the same terrain, starting and finishing with the same music, but clearly finding an easier route. Here we heard it first, like a simpler variation pointing towards more complex developments. The fact, though, is that it was written later (this was its London premiere), which suggests a clarification or elucidation. I hope that it won't be too long before we get a chance to hear the two works performed this way round.

Both, to my ears, show Birtwistle on top form. The melodies in Night's Black Bird - those long cantilenas that wind past every obstacle, as in the brilliant Theseus Game - make it instantly attractive to anyone able to hear through the huffings and gruntings of the Birtwistle awkward squad, the deep brass and screeching woodwind. The Shadow of Night is certainly a tougher nut, and not only because it's twice as long. The goings-on, as well as the techniques involved, are somewhat similar. But the discourse is so much more changeable that the ear doesn't settle (why should it?).

Its real mastery lies in the control of pace over long spans of superficially slow music. The melodies do indeed wind along slowly, but the music itself changes speed all the time, and the result is a wonderful shifting landscape of bizarre textures like a set of transparent Bosch paintings laid on top of one another and slid around. It reminded me of certain pages of Wagner, for all its ruthless rejection of sumptuousness. But it's a Wagner night without the love-death, and none the worse for that.