Sometimes when reviewing music college opera there’s a ticklish decision to be made: whether to encourage with unstinting praise or to follow the inexorable path of honesty wherever it may lead. Given that the student performers are on the threshold of professional careers I prefer the latter, albeit with names withheld to protect the guilty.

Not that of Martin Lloyd-Evans, though, the man whose billing as Guildhall’s ‘resident producer’ implies a wider brief than just directing. His credentials as a major talent are underlined season by season at Opera Holland Park, where La rondine, Die Fledermaus and two-thirds of Il trittico are just three of his recent triumphs. Why, then, is his student Poulenc so ropey? I’ve no idea.

Dialogues des Carmélites, an opera based on events during the paranoid Terror that followed the French Revolution, uses an abridgement of a play by Georges Bernanos as its libretto. The tale of Blanche de la Force, a mentally fragile aristocrat who seeks solace and refuge from the real world by joining the Carmelite community of nuns, is in some ways a companion piece to Debussy’s Pelléas et Mélisande. The material and style are different but the two works share characteristics that place them within an extended family of French opera: both composers set conversational text with a free yet hypnotic pulse; each tells a visceral tale with fragile, almost humdrum control.

While everyone’s French pronunciation was magnificent (kudos to the Guildhall language coaches) it’s a mistake to sing Dialogues without expressive colouring, and several of the Guildhall singers fell into that trap. There was nothing anaemic about Francis Poulenc: he was a full-on composer whose music is never short of a kinetic rush, so this static staging was misconceived because time and again the music cried drama while the characters stood still. All that plus an extra black mark to conductor Dominic Wheeler for allowing some students to deliver it like latter-day Gregorian chant. At their first rehearsal they should have been exposed to La Voix humaine to help them unlock Poulenc’s musical language. In Dialogues des Carmélites, notwithstanding the pacific nature of so many characters, the undercurrent is every bit as passionate as that monodrama.

Lloyd-Evans has set his production in period but his aristocrats lacked bearing and his soldiers lacked menace. Crowds failed to seethe; nuns were shorn of character. There were notable exceptions - Chloë Latchmore’s Mère Marie was a psychologically complex creation and Claire Lees crafted Soeur Constance as a naïve optimist (both stood out vocally, too) – but too often character development began and ended with the costume – and not always there, given the preponderance of 21st-century shuffling on display. As Blanche, Lucy Anderson was more at ease in her nun’s habit than the wig-and-bustle of her early scenes, although her rich, focused soprano proved she had the measure of the role. And due credit to Georgia Mae Bishop who threw herself heart and soul into the death throes of Mère Jeanne, the Prioress.

Evocative abstract sets by takis, exquisitely lit by Robbie Butler, lent a professional touch but seemed at odds with the literal approach that Lloyd-Evans espoused throughout the opera. Throughout, that is, until the overwhelming final scene: the Salve Regina that begins as a choral outpouring and ends as a solo as the nuns are guillotined one by one. Here, rather than remain true to his prevailing naturalism the director opted for a cop-out and the most moving scene in 20th-century opera went for nothing as nun by nun the sisters stepped forward for what appeared to be kit inspection. I’m still puzzling that one out.