Panic at the Disco, Astoria, London

For a band that has mislaid much of its original fanbase, this Las Vegas foursome remain in exceptionally high spirits. Panic At The Disco were at the forefront of emo, and responsible for some of the genre's most irritating habits – not the supposed interest in suicide, but the pretentiously long song titles, rambling verbiage and whining.

On their 2005 debut, A Fever You Can't Sweat Out, they delivered all that in unrelenting, hyperactive fashion. A year later, at the Reading festival, singer Brendon Urie was hit by a bottle and briefly knocked out. Recording the follow-up to Fever, Panic scrapped what had been done and began afresh, coming up with Pretty. Odd., a kaleidoscopic mix of orchestral pomp and Monkees cheeriness.

Its sleeve art forms tonight's backdrop and references The Small Faces' Ogden's Nut Gone Flake rather than My Chemical Romance's The Black Parade. The band themselves have lost the exclamation mark that once went after Panic, the circus performers they took on tour and, judging by sales, one and a half million fans. Those who remain yell back every line and scream in between.

Two of the band, Urie and guitarist Ryan Ross, have gained mop tops, but there is more to their evolution than mere image change. Panic's twitchy vocalist is flanked by garrulous bassist Jon Walker and the more severe, though no less talkative, Ross. "Behind the Sea" is transformed into swinging cosmic country, while "Pas De Cheval" leans more towards The Divine Comedy than Busted.

"Folkin' Round" is delivered straight up as a hillbilly stomp, and only the glam racket of "Mad As Rabbits" sounds sludgy without its brass parps and pinpoint production.

In such company, older material becomes more accessible. A vibrant "Martyrdom and Suicide" (an abridged title) reminds us they were never completely about moping. A solo Urie kicks off the encore with an acoustic rendition of "Time to Dance", something you could not imagine in their stilted and defensive former guise. With infectious glee, he is already starting the next number as his band mates re-emerge. Downsizing has rarely been such fun.