Imogen Heap and the Holst Singers at the Roundhouse

It isn't often you hear six members of the Estonian Television Girls Choir join forces with the Holst Singers and Imogen Heap to perform a live a cappella soundtrack to a 1920s French surrealist film about a libidinous priest. But I am delighted to have seized the opportunity last night, when this bizarre setup hit the Camden Roundhouse as part of this year's genre-baiting Reverb festival. Also featuring the emerging singer/composer Ana Silvera, this was a spectacular and otherworldly evening of strange sounds in odd contexts.

The main feature was a score by Imogen Heap played live to accompany director Germaine Dulac's 1928 film The Seashell and the Clergyman. The piece premièred in March last year as part of the Bird's Eye View festival at the Queen Elizabeth Hall, and is an imaginative and bold take on a film at least its equal in terms of unexpectedness. Occasionally said to be the first surrealist film (shown a year before Luis Buñuel's more famous Un chien andalou), The Seashell is more-or-less about a clergyman in love with another man's wife, following his hallucinations and imaginings in a decidedly non-linear manner. Effects abound, from the odd splicing of a man's head to the frequent dissolving and fading of images. Any meaning which does emerge is strongly downplayed.

Heap's score was masterfully realised by the Holst Singers, plus their Estonian guests, under the direction of Hugh Brunt, and with Heap herself taking a lead part. The film's oddness was channelled into a range of extended techniques, including vocal slides, percussion effects and the occasional intriguing vowel sound. Free-flowing and wordless, it swirled around the visuals effectively and often showed an impressive compositional grasp of structure, building to moments of tension and deploying silence and softness with skill.

On the other hand, this did not feel like the work of an experienced film composer, but rather a project by a highly-skilled musician used to working in another medium. Imogen Heap's albums and songs are fantastically inventive works in their own terms. But the compositional style of her score was problematic when placed against the film. Too often, she was reliant on bouncy pop-style ostinati and traditional harmonies, which stood in stark contrast to the dark, expressionistic machinations of the cinema. While Heap showed great inventiveness in terms of the techniques she asked the choir to perform, there was a lack of engagement with the murkier harmonies and weirder formal juxtapositions of musical avant-gardism which this uniquely dissonant film surely demands. There was also a considerable surfeit of 'Da-da-das', which is a bit of an a cappella 101.

All in all, I came away from this performance more intrigued by the film than the soundtrack – which is, of course, much as it should be. But the presentation of the work as a showpiece for Imogen Heap was not, I don't think, a complete success. This is, though, only despite the undoubted imagination and intelligence of the piece; it is refreshing and exciting to see collaboration occurring not only cross-medium but also cross-genre, and long may such projects continue. What would be truly boundary-pushing, of course, would be for Heap to release this half-hour-long soundtrack as a single.

Before Heap took the stage, we were treated to two sets from Ana Silvera with the Estonian Television Girls Choir. They played Oracles, a seven-piece song-cycle, and Step Onto the Ground, Dear Brother!, an experimental four-part piece which ruminated on 'Mystery', 'War', 'Death' and 'Love'. Big concepts, which received a big performance, and these elegant pieces were enthusiastically received. Much like Heap's piece, the music was rooted in pop harmonies and concepts, but not restrained by them, often using rich, post-romantic harmonies. But again, boundaries would have been pushed further if the Heap performance had been prefaced by, for instance, fifty minutes of Elliott Carter.

The show was to a large extent stolen by a stellar performance by the Estonian Television Girls Choir, who returned after the film for a set of pieces by Veljo Tormis. The last of these, Sampo tagumine, was choreographed as well as sung, and was almost unbelievably well realised. With brilliant control of tone and dynamics and exquisitely precise movements, this was a performance of amazing passion and stunning discipline, which four years of evensongs at Cambridge had not prepared me for. My hat goes off to them and their director Aarne Saluveer.

This was precisely the kind of inventive and worthwhile project which Reverb is making its own, and I am greatly looking forward to returning to the Roundhouse next weekend for more. It's worth adding, though, that this could have pushed its genres more: it was faithful throughout (save when the Estonians were centre-stage) to a fairly well-established 'arty pop music' aesthetic which could have been challenged or subverted by juxtaposition either with more conventional pop music or with more conventional classical music. As it was, the evening stood fairly securely in an interim position, enjoyable but not completely revolutionary.