Transit New Music Festival, Leuven, Belgium

Last weekend, Belgium basked in the same glorious autumnal sunshine as Britain, helping to make Transit, Leuven's annual new music festival, a most uplifting experience. Its location is a beautiful old university town close to Brussels. Transit – now in its second year, directed by Mark Delaere as part of the large-scale Flanders Festival – is quite modest in scope: just a weekend of concerts and seminars. Yet it is planned with ambition as well as imagination, and audiences are enthusiastic.

Three ensembles made an appearance, the first two of them Belgian. The well-established group Champ d'Action, conducted by Michael Finnissy, seemed a bit below par; Apsara, on the other hand, were as much to be congratulated as the composers they played for, making a recital of music for recorder quartet and percussion vastly more compelling than I'd feared.

The excellent Frankfurt-based Ensemble Modern performed the final concert under Kasper de Roo. The musical highlight of these three programmes was probably the last-named group's account of Kaija Saariaho's Graal Théâtre, with Jagdish Mistry a fine violin soloist. But the underrated American composer Alvin Curran's Four Flukes or Feldman's Follies proved a most engaging piece for a variety of recorders and improvising percussionist. And on the basis of his Knochen, a work of vivid contrasts and sometimes entertaining timbral inventiveness, I'd recommend British ensembles to explore the music of the young German composer Enno Poppe.

Dominating the proceedings, however, was our own Michael Finnissy's The History of Photography in Sound for solo piano, played by Ian Pace. The 11 "chapters" of this – the latest and largest of the composer's cycles of piano music, written between 1997 and 2000 – last five and a half hours; with breaks, this performance (only the second time, following a performance in London early this year, that the work has been played complete) began at 1.30pm on Saturday and concluded around 10.30pm.

Finnissy uses the word photography, he says, "and its plethora of associations, to convey a certain kind of musical material: documentary – snipped out from different periods in the past, and different locations across the world – a collection of exterior facts. These refugee facts are then situated, more or less provocatively, in the eventual composition." Quoting everything from Bach to a variety of folk musics, the result is a work that seems about much else – autobiography, for instance – beyond its musical materials and processes themselves.

Provocation is also one possible reason for the work's sheer length, of course, as well as its formal and, often (though gratifyingly not always) textural complexity; there were times – notably, during chapter eight, entitled Kapitalistisch Realisme (met Sizilianische Männerakte en Bachsche Nachdichtungen), which lasts 70 minutes – when my ability to keep track of musical events was tried almost beyond endurance. But by then I'd not only already been grappling with Pace's performance for much of the previous six hours, but also with an illuminating two-hour seminar on the work that morning, presented by Christopher Fox.

I accordingly hope that Pace and others will continue to programme individual chapters. Some certainly yield their secrets more readily than others: chapter six, for example, Seventeen Immortal Homosexual Poets – the cycle's central axis – is easier to follow in harmonic as well as gestural terms than much of the rest, and it includes some of Finnissy's most beautiful music. With several chapters, however – and provoked into a different kind of listening by a short talk of the composer's own before chapter five – I occasionally found the music's obsession with collage-like structures, likened to what he called "display and denial", frustrating rather than the window onto fresh modes of listening that they are clearly intended to represent.

All in all, however, Finnissy's The History of Photography in Sound is an astonishing postmodernist achievement; the vividness of its writing for the piano is in itself a virtuosic accomplishment. Pace's achievement – mental as well as sheerly physical, and in variety of touch as well as deepening contrapuntal intelligence – was astonishing too.

Nor was this all, since some portions of the work were acc-ompanied by slide projections or video, provided by Ken Scott and Anouk De Clercq; these evoked many associations, like the music, but not ones always readily linked to the aural experience. Imaginative though some of these images were, I found them distracting more often than illuminating.