It is no easy task to find the right acoustic for a work such as Britten's War Requiem. The widely contrasting textures from chamber to full choral and orchestral with expressively vital interjections for distant boys' choir suggest three different types of resonance, and certainly the performance in Westminster Cathedral on Wednesday evening by the Royal College of Music Chorus and Symphony Orchestra under Steuart Bedford was only intermittently favoured by the building's vast spaces.

The occasion marked the 20th anniversary of the composer's death, and revived the reconciliatory aims behind the work's 1962 world premiere, which celebrated the rebuilding of Coventry Cathedral after its destruction by enemy bombing in 1941. Russian, German and English soloists were employed, for instance, representing the war's chief combatants, as Britten originally intended (the originals being Galina Vishnevskaya, Dietrich Fischer-Dieskau and Peter Pears), while the big choir included guests from all over Europe and Scandinavia. Everything pointed towards the fact that the moral impact which Britten had intended, focusing all of his mighty technical armoury in the process, was very much in the performers' minds. This was not to be a solely musical experience.

In the event, much of that seriousness of purpose and commitment came across in an interpretation that, despite the acoustic problems, was powerfully focused by Bedford. The complexity of Britten's vision stood little chance of being fully revealed, however, since it was only the more massive choral and orchestral textures and the celestial writing for distant choir (superbly rendered by the cathedral's choristers) that were able to bloom acoustically.

The settings of Wilfred Owen's war poems - which, with their subtly inward chamber accompaniments, are interwoven with the Latin text of the liturgy - were, as far as one could tell, sensitively sung by the tenor Mark Wilde and the baritone Johannes Beck, but tended to be overwhelmed by the building's presence and often failed to carry beyond the first few rows. And yet, somehow, the essence of Britten's vision was able to leave its distinctive mark.

Naturally, the simpler the harmonic texture, the clearer was the expressive message, and the eerily moving setting of Owen's "Strange Meeting", evolving over timeless harmony, was splendidly realised. Likewise the glorious celebrations of the "Sanctus" and "Hosanna", with Iana Ivanilova's radiant soprano soaring aloft. But when Britten gathered his thematic and textural forces in the final "Libera Me" for the work's climax and most complex musical statement, there was much we had to take on trust. The great G minor peak of this war-torn musical landscape still made its appalling effect, nevertheless.

Throughout the evening, Bedford galvanised his vast forces, and the choir sustained its singing commandingly, whether focusing attention on the tritonal cadences that so movingly bring F major connotation or hurling forth majestic waves of sound elsewhere. The full orchestra and chamber ensemble contributed no less powerfully.