Robyn, V Festival, Chelmsford<br/>Mark Ronson, V Festival, Chelmsford

He ought to know better. Last time James Dean Bradfield tempted fate by singing "Raindrops Keep Falling On My Head" at a festival, the heavens opened. Today, the Manics man announces "This song is a kind of incantive prayer for all the people of this fabled isle... wishing you an 'Indian Summer'!", and immediately, monsoon season returns to Chelmsford.

You'd think that the masses fleeing the downpour at the Channel 4 stage might boost the crowd in the Virgin tent next door, where Swedish pop pixie Robyn is about to appear, but the place is strangely empty.

Seeing a No 1 single performed by the artist when it is top of the charts ought to be a special, spine-tingling moment. Seeing the No 1 single performed in front of, er, no-1 is an awkward, uncomfortable one. The reason that the V crowd hasn't flocked to see Robyn sing "With Every Heartbeat", her chart topper with producer Kleerup, is apparently a matter of billing: the organisers still have The Bravery listed for this slot. (If I were them, I'd be worried.) She'll survive the setback. She's managed so far.

A child star in Scandinavia, she reached worldwide attention aged 16 with "Show Me Love", a fairly average piece of techno pop, then faded from view. Rather than settling for anonymity in Stockholm, she regrouped, reinvented herself as an autonomous electro diva (pitched somewhere between Peaches pugnacity and Miss Kittin serenity), resulting in the wonderful album Robyn.

Surveying the two-thirds empty space, Robyn and her minimal backing band kick off with "Cobrastyle", that album's finest moment. Hand on pleated hip, the 28-year-old has a Larry David "pants tent" to go with her boyish blond haircut; Robyn's androgyny goes beyond her forename. Her voice is surprisingly soulful (under the regulation rockist definition, ie wavering), her demeanour defiant. "Here's a song from 11 years ago," she teases, then surprises everyone with "Show Me Love" itself.

It's "With Every Heartbeat", sublime and transcendent, a hybrid of hands-in-the-air trance and Europop tragedy, which makes you forget you're standing ankle-deep in slurry and facing a three-hour journey home. Sod Rihanna and her umber-ella-ella, this is the single of the summer.

Who is Mark Ronson? Ronson's dad Mick was the Spider from Mars who was famed for receiving a mimed blowie from Bowie, and lending his prolific panache to half a decade's worth of inspired and immortal rock songs. Ronson Junior, however, is in danger of becoming best known for being a well-connected schmoozer, a musical Perez Hilton who can call on an A-list of celebrity friends to help out on whimsical cover versions.

It wasn't always this way. Ronson's debut album Here Comes the Fuzz, an original work, gained him respect as a hip-hop producer. It was this year's Version, his commercial breakthrough, which did all the damage, with its jazzy, bluesy, loungey remakes of contemporary hits.

Not that Version didn't have its moments. Its hit rate was roughly one in three. But in concert, even those fall flat. None of the A-list joins Ronson today, leaving the project looking somewhat denuded.

Phantom Planet's Alex Greenwald spends an extended spell on stage with a band who all have their names on their T-shirts (so that Ronson can remember their names?), as does rapper Wale (pronounced Wah-Lay), a Ronson protégé from Washington DC.

We are also treated to the lettuce-limp Candie Payne, wearing the kind of high-waisted tartan shorts Simon Cowell might buy for a golfing holiday, whose bafflingly easy rise becomes less baffling when Ronson mentions that her brother is in the Zutons. Nepo-tastic!

The Zutons' "Valerie", one of the few highlights of Version, is performed by south London newcomer Tawiah due to the continuing "difficulties" of Amy Winehouse. One Version star who does show up is Daniel Merriweather, who looks nothing like he sounds. A chunky ginger guy from Melbourne in a floral shirt whose sleeves appear to have been fashioned from the upholstery of a particularly nasty seaside hotel, he re-enacts his murder of "Stop Me If You Think You've Heard This One Before". Stop him.

You're left wondering why Mark Ronson has put so much effort into assembling a bog-standard covers band. At the end of his "best gig ever" (seriously?), Ronson wryly adds, "We'd like to play for seven hours, but we don't know enough of other people's songs."