Here's a salutary lesson for directors and their production teams: if on first night you hop on the stage for the curtain calls and trip over the set, just imagine how it must have been for the cast, singing their socks off for two hours while picking their way round your idiotic obstacle course.

In their Glyndebourne debut with the late Mozart opera La Clemenza di Tito, director Charles Guth and designer Christian Schmidt also fell into that trap of thinking that the composer needs some help: that his story of boyhood friends turned adult foes is not clear enough. That Rome needs to be moved to a present-day reed swamp. But ha ha: the reeds hit back.

We are in Rome for the brief reign of Titus, who finally chooses for his bride Vitellia, but not before she has despatched her lover Sesto to kill him. Sesto, rumbled, takes the blame for the conspiracy, Vitellia owns up at the last, and Titus dispenses the clemency of the title by forgiving everyone, but much good does it do him, as in this reading Clive Bayley's pushy, impressively sung Publio assumes the crown.

Almost nothing in this production adds or illuminates one jot. Many tropes are tired – that the scheming Vitellia smokes equals a Very Bad Person. Her love affair with Sesto opens the opera and gosh, there is simulated sex. There are stifling back projections of two boys killing a magpie (one for sorrow...). Sigh.

Fortunately this clumsy nonsense is outshone by wondrous singing from a hastily re-formed cast. The original Tito and Sesto pulled out of the production, leaving splendid British mezzo-soprano Anne Stéphany to step up from her original role as Annio to become a very brilliant Sesto, a trouser role, while Richard Croft was hastily enlisted in the title role with a performance of fragility and humility that belied its short order.

Alice Coote's Vitellia looked peeved, but then, she's the sort of heroine who points a gun at her lover's head to encourage him to kill another man for love of her. I'm pretty sure sure that was not in Mozart's plan.

With Michèle Losier also stepping in at the double, as a box-fresh Annio, nothing is stale except the directorial ideas. There are glorious musical moments, including three marvellous trios, although the final sublime chorus was marred on first night by a sluggish entry. And why are men and women alike, with the gestures that director Peter Sellars dreamed up 20 years ago (they were great then, in this house's Theodora), in lounge suits? Could it be because the entire production team is male?

Glyndebourne music director Robin Ticciati has nimble forces in the Orchestra of the Age of Enlightenment, and Antony Pay's basset horn obligato is one of those moments you want to replay in your head for ever.

When the house lights are brought up for Alice Coote's revelatory last aria, 'Non piu' di fiore', which the instrument accompanies, it briefly brings the singers closer to the audience. Otherwise, they are in a wilderness. And the production team is in cloud cuckoo land.

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La Clemenza di Tito is sung in Italian with English surtitles. It is relayed live in cinemas across London on 3 Aug. Click here for details and booking.