Lampchop, Somerset House, London

Can gigs get any more civilised than this? In the stone courtyard of the grand Georgian edifice and the former home of the register of births, marriages and deaths (and the Inland Revenue), Nashville's most humane and social-minded country-soul revue, Lambchop, seem right at home. It's a clear summer evening, the air so still that squawking seagulls overhead are a factor in several songs. But it's Lambchop's 11 musicians (a conservative turn-out – sometimes there are upwards of 30) who create most of the atmosphere.

Dressed in colourful, well-tailored suits and dresses, they look like a respectable young band in 1967. As was the case when they filled the Royal Festival Hall last year, on the back of their fourth and by far most successful album, Nixon, the first thing they do is look at the crowd with undisguised delight, taking snapshots for the folks back home, astounded they have such a following. They then play a near-two-hour set, composed almost entirely of unrecorded songs that no one here has heard, and there isn't a murmur of protest.

One reason is that the singer Kurt Wagner's songs are consistent in their elliptical, spiky, haunted evocations of domestic trauma and disappointment, while his band have found their sound. A new song, "The Daily Growl", is typical: seeming to address something wrong with America and its "emotionally challenged" men, while seven guitars, a piano, a vibraphone and a synth sounding like a ghost howling up a mine-shaft complete the musical picture. Nixon's "You Masculine You" then proves the emotional range this band can travel in a single song. Wagner sings of more troubles in a pretty falsetto, while the musicians play lulling chords, softening his pain – until the singer, eyes closed and mouth cranked open, starts to bark and scream, and Lambchop erupt around him.

They function like an old-fashioned, disciplined jazz band, pulling away to investigate each song before reliably returning to its melody. Wagner's moods also shift, from a whimsical tour tale to a tormented noir lament, a jaunty Sixties love song to a disposition on shoddy toilet habits. "I lay there wet and naked/ and I know you heard me yelling," he accuses one minute. "Uncles, uncles, uncles... fester," he muses the next.

The most civilised aspect, though, is the relaxed, egoless relationship between band and audience. When Wagner tunes his guitar the bassist tells a few jokes, and there's a general, unspoken air of politeness, amusement and decency. The Nixon anthem "Up With People" catches that communal spirit and turns it into rock'n'roll as the crowd clap and cheer along, and Wagner lets out a last, hoarse shriek. This is the last date of a short UK tour. It's been quietly heart-warming.