There is no doubt that Niccolò Jommelli's opera Il Vologeso, set in 2nd-century Ephesus, has an action-packed plot. Within minutes, King Vologeso IV, defeated and presumed dead, turns up at a banquet disguised as a slave and attempts to poison Lucio Vero, the victorious Roman emperor. But the emperor passes the goblet to Berenice (Vologeso's bride) causing the “servant” to dramatically confess his crime. Lucio, who lusts after Berenice – despite being betrothed to his co-emperor's daughter – is furious and has Vologeso clapped in chains. When he is thrown to the lions, Vologeso denounces Berenice, who protests her innocence by leaping into the arena to face death with him. The emperor is conflicted but throws Vologeso his sword so he can slay the lion and thereby save Berenice. But, Lucio's actions get him into hot water with fiancée Lucilla, who's rather inconveniently just pitched up from Rome. (I do hope you're keeping up at the back.)

The trouble is, all this action is confined to recitative which – in a concert performance such as this – rattles past at lightning speed with only the seasick-green Cadogan Hall surtitles to guide members of the audience who failed to fully digest the synopsis beforehand. Blink and you miss it. Despite cuts made by Ian Page, Artistic Director of Classical Opera, Jommelli's opera is as long as it is convoluted. It was presented here as part of Mozart 250, a 27-year project which attempts to put Mozart's work into context. Jommelli, born near Naples in 1714, composed around 80 operas. Il Vologeso was premiered in 1766, while he was Kapellmeister to the Duke of Württemberg in Stuttgart. It nods towards Haydn and Gluck, with a definite Sturm und Drang influence, but also harks back to Baroque opera in its structure: sequences of arias with ensemble numbers – a quartet, trio and sextet – to end acts. An interesting feature is that while most of the recitatives are backed by a continuo group, some scenes are accompanied by full orchestra. As Berenice is faced with a terrible choice – to accept Lucio Vero's hand in marriage or to face Vologeso's severed head – icy tremolando strings add their commentary.

Jommelli's score provided quite a showcase for the six young singers, who responded with admirable spirit. Despite having the title role, there were limited opportunities for Rachel Kelly's Vologeso, who spends much of the opera in and out of prison like a revolving door, flitting between fury and despondency. Her bravura aria “Invan minacci” displayed an heroic, bright upper register and agile runs, while her long crescendo on the first syllable of “lasciami” in “Cara, deh serbami” gradually bled warmth into her voice – a nuanced, dramatic portrayal.

Stuart Jackson's Lucio Vero demonstrated that he is developing into a fine tenor. He was not at his best in his opening aria, cutting phrases short, his tone bulging during florid runs, but he recovered well in the fiery, Gluck-like “Sei tra' ceppi”. Jackson whitened his tone dramatically and also had an almost baritonal lower register. His tempestuous aria “Uscir vorrei d'affanno”, a bit of a Baroque throwback, displayed blustery coloratura, but was energetic and finished strongly, with plenty of snarl in the voice, presenting Lucio's conflicted character well.

Jommelli's opera is sometimes subtitled Berenice, Queen of Armenia and Gemma Summerfield, especially in a busy Act III, dominates much of the action. Her soprano hardened a touch towards the top, but she did indignation and scorn well. Angela Simkin's darker mezzo cut a noble figure as Lucilla, singing with beautiful warmth, while Tom Verney sounded underpowered as Aniceto, Vero's attendant, although he was on the far side of the hall.

Jennifer France is a diamond of a soprano! She sang Flavio, the Roman ambassador who arrives with Lucilla and leads the revolt that ultimately forces Lucio to cede Berenice to Vologeso. She only gets one aria, “Crede sol che a nuovi ardori”, but grabbed the opportunity with both hands! A bright, alert presence, she surveyed the hall confidently during the orchestral introduction, then pinned us to our seats with gleaming tone, fabulous hairpin dynamics and a controlled trill – a showstopper!

The Orchestra of Classical Opera had a mixed evening. The Haydnesque overture came off well, pairs of flutes and oboes given much pastoral work, but later there was queasy string tuning at times and agricultural playing from horns and oboes all but sabotaged Berenice's Act III scena.

A staged performance is always going to present a better case for Il Vologeso. Those curious to discover more could head to Stuttgart next month.