You're familiar with the pub session - those beer-fuelled bedrocks of the folk scene at which a resident band is joined in a knees-up by singers from the floor.

But imagine what it would be like if you could capture the informal, all-pitch-in spirit of the pub session and bring it to the big stage.

Imagine if the resident band were the finest folk veterans of the past 25 years and the guest singers were a mix of old icons and the best of the new wave.

Oysterband have not only imagined it, they've made it happen, creating a fresh take on performance in which everyone picks what they want to sing and, if you know it, you join in.

Having involved such luminaries as famed fiddler Eliza Carthy, electro-folkster Jim Moray and scene stalwarts Show Of Hands, their Big Session is now on its third, album-accompanied tour - the Brighton leg of which will feature June Tabor, Singer Of The Year at 2004's BBC Folk Awards, Irish piper James O'Grady and cult gothic duo The Handsome Family.

"For this third one, knowing they wanted to make a record, we thought it was safer to involve some old mates like June Tabor," says Oysterband's violinist Ian Telfer.

"Then we thought, 'maybe this is too predictable', so we deliberately set out to involve somebody who we didn't know at all, and that was The Handsome Family.

"They certainly have at least one foot, possibly both feet, in the British folk tradition," he observes of the alt-country stars. "Renne, who writes all the words, is most definitely into the blood, thunder, gore, incest, murder, wolves, madness, in-the-forest side of life. And that is strong in folk tradition."

An experiment in the strangeness and diversity of the flowers you can grow from the rich, dark earth of traditional music, Big Session live shows involve a mix of covers and originals, with the guest singers and musicians selecting from their own and each others' back catalogues or teaming up for new takes on traditionals.

"CDs and MP3s are flying across the planet even as we speak," says Telfer. "I'm all in favour of going out on stage without knowing exactly what you're going to do but you can't be totally impromptu - you'd end up going for the lowest common denominator and just playing The Wild Rover."

"You're never quite on top of what the arrangements are going to be though," he laughs. "What seems clear as crystal in the afternoon suddenly becomes rather vague and mysterious at ten o'clock at night."

From The Handsome Family's take on traditional The House Carpenter (they chose the American version because it had more "unexplained violence") to the full-band, electric-guitar driven strains of Oysterband's own song We Shall Come Home, the live album promises much. But the best thing about doing the Big Session, says Telfer, is the moments - like June Tabor and Oysterband frontman John Jones' take on Joy Division's Love Will Tear Us Apart - of sheer unpredictability.

"There were something like 88 covers of that song in existence in the month we did it," he says, "so the world certainly wasn't crying out for another one.

"But, on the night, they really made it work as a beautiful, fragile duet. There was just an indefinable atmosphere."

Starts 7pm, Tickets £15, Tel 01273 709709