This Prom was a fine example. The all-Mozart programme might have been cunningly designed to show off the composer’s emotional range, beginning with the gentle comedy of the concert aria Per Questa bella mano. The text’s plea of undying amorous constancy seemed sincere enough on the page, but Mozart clearly didn’t take it very seriously.  He surrounded the vocal melody (eloquently sung by bass Hanno Müller-Brachmann, standing in for Neil Davies) with an absurdly virtuoso double-bass part, tossed off here by the Budapest Festival Orchestra’s principal bassist Zsolt Fejérvári. The concert then moved by gentle degrees towards Mozart’s solemn side. His Clarinet Concerto is one of those pieces that’s played too often for its own good, but in this performance, played on an appropriately deep-toned basset clarinet, it came up fresh and new. Soloist Ákos Ács did this with unshowy, graceful musicality, which was mysterious and eloquent just because it was so simple on the surface. Much the same could be said of the orchestral playing, which in the closing performance of Mozart’s unfinished Requiem took on a deep, inward expressivity which could truly be called sublime. There were no distracting “period” mannerisms, and conductor Iván Fischer decided to stick with the traditional completion of the Requiem by Mozart’s friend Süssmayr.  Its superiority to the various modern scholarly completions seemed especially evident in this performance, thanks to the tenderly expressive glow the four soloists gave to Süssmayr’s portions of the Requiem, such as the Benedictus. When these qualities were joined to Mozart’s genius, in those portions of the Requiem that were indubitably by him, the effect was quietly overwhelming. Fischer’s tempos were often unfashionably slow, but the phrasing he coaxed from the orchestra was so shapely that they never seemed sluggish. The choral singing from the Collegium Vocale Gent was heart-breaking in its tenderness, as was soprano Lucy Crowe when she pleaded “to whom shall I appeal for aid?” in the Tuba Mirum, a soft moment amid the minatory sternness. At the end, Fischer led chorus and orchestra to a quiet final chord  that was startling in its total refusal of the grand manner. He got his reward, in the stunned silence of the audience. Watch and listen to this Prom on the Proms website