Duffy, Somerset House, London

The annual season of concerts in the central open courtyard of Somerset House has become one of the more romantic fixtures of the London summer, offering as it does the possibility of alfresco entertainment minus the mud and the prospect of bunking down under canvas. Even when, as tonight, the squally breeze brings drizzle rather than drowsy warmth, it's never a real threat to audience spirits – particularly if the performer happens to be the year's most successful new act, her debut album Rockferry racking up in the region of two and a half million sales in the four months since its release.

Her combination of powerful voice and stunning looks has made Duffy the most instantly recognisable of Britain's bumper harvest of new female singers. She may not have quite the swashbuckling panache of the indomitable galleon Amy Winehouse, but the echoes of Sixties icons like Brigitte Bardot and Carol White give Duffy's more conventional beauty the sophisticated grace of an ocean-going yacht, her visage striking enough already to have secured her a spot on the walls of the National Portrait Gallery, confirmation of her remarkably rapid ascent.

Playing outdoors means there's no dimming of lights to draw the expectant clamour of the crowd. Instead, a lone guitarist plays herald with the intro to "Syrup and Honey", before Duffy saunters on in red heels and a navy blue minidress to wring the last soupçon of soul from the song. "I'm not going to milk it," she says coyly in the pre-climactic lull, then proceeds to do just that as the song sweeps to its conclusion.

It's a courageous start to the set, designed to demonstrate how little her huge voice relies on instrumental support to make its impact; so when the familiar piano chords of "Rockferry" announce the arrival of the rest of her band, it's almost like a bonus. Well-drilled and unassuming, they carry her smoothly through a set built around the 10 tracks of her album, plus a small handful of extras, of which the strutting funk groove "Tomorrow" offers the best opportunity to vary the pace and tone of the show, without straying too far from her blue-eyed soul roots.

She's clearly nervous, chatting garrulously between songs about the "gorgeous venue – not like in Wales!" (before conceding that maybe Cardiff has some place comparable), and introducing "Serious" with blurted patter about being someone who "hated labouring – labouring? – labelling relationships", then engagingly admitting that she still has problems with the English language (she's a Welsh speaker).

Not enough, though, to stop her introducing "Warwick Avenue" with more patter about relationships, slipping from the speech straight into an a cappella first verse, before the band punches in for the rest. It's a dramatic way of starting a song – so effective that she employs exactly the same gambit for "Delayed Devotion"; but by the time she uses it again for the set-closing "Mercy", it has the opposite effect, dissipating the impact of the distinctive organ hook that should rouse the crowd's anticipation of the big hit single.

That's not the only way that Duffy's a bit of a one-trick pony, either. Perhaps hobbled by those scarlet high heels, her stage moves are limited to a sort of slow high-step, kicking each foot back at right-angles like some human dressage pony, as she shuttles carefully from drum-riser to stagefront and back again, spending the brief instrumental breaks posing on the plinth amid her carefully arranged musicians like an act on Ready, Steady Go!. It serves as a reminder of how firmly rooted in the styles and sounds of Sixties soul she is, something confirmed when she returns for an encore of Solomon Burke's "Cry To Me", before taking her leave with the big, anthemic album-closer "Distant Dreamer".

It's an enjoyable enough evening, albeit somewhat repetitive and predictable in approach. There's no denying the power of her blue-toned pipes, but while she hits all the high notes accurately, and her vibrato on the long notes judders with exactly the right pace and depth, there's never any suggestion that she might be overcome by a sudden burst of overwhelming emotion. It's soul music as an exact science, rather than a feeling.