If I had to come up with a unifying theme for the BBC Singers’ Proms Chamber Music concert at Cadogan Hall, it would be “mystery”. Sir Harrison Birtwistle linked moths with mysteriousness in his mid-concert interview prior to the UK première of The Moth Requiem. Gustav Holst explored the mystery expressed in the ancient Rig Veda texts in his Choral Hymns of the Rig Veda. His daughter, Imogen, set 17th-century poet William Cleland’s text chronicling the mysteriously free movements of the imagination in Hallo, my fancy, whither wilt thou go?. And the Eton Choirbook, from which the remaining two programmed pieces – William Cornysh’s Ave Maria, mater Dei and Stella Cæli by Walter Lambe – are taken is nothing if not mysterious in its collection of late 15th-century English music, the magnificently florid style of seeming to appear from nowhere.

It’s a tenuous theme, and the BBC Singers’ performance made it more so. Only in the first and last works of the concert – Holst senior and Birtwistle respectively – was a sense of anything inscrutable communicated. Of course, none of the music in this concert was composed to be mystical; but music, especially choral music, has such an innate potential to transcend the everyday that when it fails to do so, one can’t help but feel a little mystified.

The Third Group of Holst’s Choral Hymns is the most performed of the four he wrote, thanks to its neat combination of women’s voices and harp. Lucy Wakeford from the Nash Ensemble joined the ladies of the BBC Singers for this evocative set of four hymns, in which Holst conjures up vivid musical imagery: the Dawn, with its rising fourths spreading gradually from low alto to high soprano; the Waters, sparkling in the morning sunlight courtesy of a twinkling harp accompaniment; Vena, the sun, rising gradually from low unison voices to a powerful, full-bodied sound; and the Travellers, whose sprightly step and song indicate the thrill of the road. Holst does not capture the depth of spirituality within these texts in the way he does in numerous hymns from Groups One and Two, but these are vibrant little pieces that are always a pleasure to hear. The Singers started serenely; conductor Nicholas Kok maintained an excellent balance in the opening of Hymn to the Dawn, holding the top sopranos back so as not to dominate the fragile texture. Later this was lost a little, particularly in the penultimate verse of Hymn to the Travellers, when the first soprano’s inverted pedal note was far too forceful. Wakeford played wonderfully throughout, and it was nice to have her in front of the choir, allowing the audience a rare glimpse into harp technique.

The Cornysh, Imogen Holst and Lambe pieces then followed as a group. This was a strange decision as they had little binding them together. The extraordinary soundworld of the Eton pieces jolted somewhat with that of the Holsts, the complex intertwining rhythms, extended melismas and melodic floridity of the former particularly at odds with the preceding Choral Hymns. Nevertheless, Cornysh’s Ave Maria was excellent, the choir handling those amazingly intricate lines wonderfully. The Lambe was a different story: the singers seemed far less confident, heads were down, and it was not a particularly engaging performance. In between came Imogen Holst’s Hallo, my fancy, a piece that has little to recommend it, really. Composed in that mid 20th-century English part-song soundworld, Holst gives each verse of Cleland’s fantasy voyage to a different voice; and whilst the sopranos’ lightness fitted the flighty nature of the music, the stodgy men did less to communicate the thrills of the free-flowing imagination.

Imagination is certainly one thing Harrison Birtwistle has in spades. The Moth Requiem is Birtwistle’s musical response to those shady creatures, and especially to species of moth that have become extinct. The moth is an emblem, said Birtwistle, of “things disappearing”, and so this piece relates to death and extinction more generally – though Birtwistle pointedly remarked that there is more anger than mourning in it. The BBC Singers’ women were joined by three harpists and flautist Philippa Davies for what was certainly the concert’s highlight. Birtwistle uses his harps imaginatively, his choir percussively and his flute soothingly, again and again countering the attack of accented syllables (taken from moths’ names and words from a Robin Blaser poem) and harp stabs with a tranquilising flute line. Yet sometimes, all three groups join forces to create an incredibly agitated music, putting the listener in nagging discomfort with a distressing sound coming from all sides. This is powerful, deep-cutting music, fiendish to perform but highly expressive, nuanced and moving; every part is virtuosic and every part was expertly performed. It was an affecting end to a concert that waxed and waned, fluttering from excellent to ordinary like the mysterious flight of a moth.