Tête à Tête, Riverside Studios, LondonProms 24, 25, 27, Royal Albert Hall / Cadogan Hall, London

A witty circus act is an unexpected pairing with a tragic one-woman opera about a medieval mystic

The third night of Tête à Tête's annual festival at Riverside Studios opened with spangles, smiles, suicidal urges, a singing horse and what director Bill Bankes-Jones called "more of a rehearsal than a performance".

Unveiled last August as a five-minute "seed-bed work", Michael Henry and Adey Grummet's big-top tragi-comedy Circus Tricks was now in the second stage of its development; with two extra characters, 25 extra minutes of music, a thrifty, witty staging, and an opening devised that afternoon.

It's not every day that you take your seat by walking past the conductor (Tim Murray) while the singers do their stuff. Henry and Grummet's contrapuntal, neo-classical, vamp-til-ready repetitions of the imperatives of circus life –"Smile! Turn! Bow! Present!" – served their purpose well, illustrating the formulaic nature of work in which one false move can be fatal.

Caught in the spotlight, Tanya the knife-thrower's assistant (Grummet) counts through her routine over carnival figures for flute and tuned percussion, breaking off to confess her desperate love for Jack (Henry) and conceive a tragic death at his hands in the blindfolded climax of their act. Poised on the platform above them, trapeze artist Alice (Lara Martins) explores the allure of falling in sugared glissandi, candied trills and citrus-sharp arpeggios, while Barney the dancing horse (Kevin Kyle) tosses his feathered headdress and spits out his bit, dreaming of an escape from the monotony of trotting "round and round and round and round and round and round and round".

From Shorts (1999) to Circus Tricks, Tête à Tête's trademark has been lightness of touch: a genial, playful, paper-moon style where comedy is deftly spiked with tragedy, and tragedy comes laced with laughter. Odd, then, that The Song of Margery Kempe (appetisingly billed as "Alan Bennett's Talking Heads meets The Exorcist) should be so indigestible. Written for Lore Lixenberg, the charismatic coloratura comedienne, Brian Inglis's one-woman unaccompanied opera is based on the ghost-written memoirs of the mediaeval mystic, whose possession by snarling demons after giving birth to her first child would today be diagnosed as post-natal psychosis. Kempe's baby and the 13 others that followed are absent. So too are her pilgrimages to Rome and Jerusalem, and her audience with Julian of Norwich. Inglis conveys the horror of living as a hate-figure but does not address the issue of whether Kempe was inspired or insane. The score sounds like a 1950s motet, minus the organ, and the only light in this dank, dramaturgically-challenged work is borrowed from the plainchant, Veni, Creator Spiritus.

Thankfully, Lixenberg and Grummet's Emergency Recital with Richard Thomas – an hour of expletive-laden epithets set to music as sweet-tempered as Tony Hatch's Downtown – sent me home with a smile.

An audience little bigger than that at Riverside stayed for Tuesday's late-night celebration of Harrison Birtwistle's 75th birthday with the London Sinfonietta and David Atherton (Prom 27). Emptied of crowds and the portentous cannonades required by Berlioz (Proms 24 and 25), the Albert Hall got its untameable acoustics in order, allowing every detail to register, from the vigorous hocketing of Carmen arcadiae mechanicae perpetuum to the sublime tonal murmurs, flickering triplets and bombastic rallentandi of Silbury Air. The shawm-like shrieks and blunt antiphons of Verses for Ensemble remain unconvincing, but the playing was immaculate, eloquent, easy. Perhaps there's a case to be made for more small-scale Proms?

Earlier that evening, Thierry Fischer's delicate, period-style performance of Mendelssohn's First Symphony and Violin Concerto with Isabelle Faust and the BBC National Orchestra of Wales (Prom 25) had more colour, wit and texture than Heinz Holliger's dyspeptic (S)irato and the five excerpts from Prokofiev's bludgeoning cinemascope treatment of Romeo and Juliet.

Snug in Cadogan Hall (PCM 3), the Belcea Quartet navigated Haydn's lethal F sharp minor quartet (Op 50, No 4) with vivacity and elegance, and explored the ambivalent landscape of Britten's String Quartet No 2 with impeccable musicianship.

Tête à Tête (020 8237 1111) to 16 Aug