The Sixteen turn 45 this year, marking nearly half a century of bar-raisingly good choral singing under their founder and conductor Harry Christophers. They’re a group to banish memories of hymns mumbled at school assemblies, massed wobbling in parish halls and the megawattage of a professional operatic chorus with a single exquisitely shaped phrase. Their brand of choral sound is lean (the clue’s in the name, though don’t count too obsessively), bold, polished to an exceptional sheen, blended as if alchemically distilled.

Or at least it can be. There were flashes of that unequivocally world-class quality in their performance of 20th-century French-language choral works, all haunted by the musical past. Opening the programme, the bare octaves and fifths of Frank Martin’s Cantate pour le 1er août were harsh in their brightness. What followed was practically translucent, the tone of 20 voices absolutely unified, every word audible. In the first of Maurice Duruflé’s unaccompanied Four Motets on Gregorian Themes, phrases were beguilingly long, then longer, then seemingly infinite – presumably through staggered breathing, albeit imperceptible from my spot in the fifth row. The beauty of such moments, of human voices drawn out in astonishingly long skeins, cannot be overstated.

Elsewhere, though, there were problems. Francis Poulenc’s choral writing can be fiendishly difficult but mustn’t sound it. The opening of his Un soir de neige revealed an unusually unblended soprano section, some of their later entries – high, quiet, weird – less than confident. (The fact that the lineup on stage differed considerably from the ensemble’s listed core members may not have helped.)

In the second half, Duruflé’s Requiem offered both the best and the worst of the evening. Baritone Simon Grant brought a coppery tone to his solos but was overstretched in the higher passages; mezzo-soprano Nancy Cole sounded uncomfortable in her solo’s low tessitura. Organist Robert Quinney provided injections of vibrant energy but also passages of relentlessly loud, reedy grit, ill-matched to the Sixteen’s mohair-soft tone. But there were also vocal lines barely ruffled by passing consonants while diction remained lucid, thrillingly full-throated climaxes and ideally paced cadences – all superintended with quiet elegance by Christophers.