Jaga/Mercan Dede, Wychwood Festival, Cheltenham

What's the Norwegian for 'blimey'?

When it comes to the personae adopted by various bands, one can admire the aesthetic appeal of lofty arrogance or ironic detachment, but honesty really is the best policy. The Norwegian 10-piece Jaga (two of whom are pictured right; they appear to have dropped the "Jazzist" from their name) don't have a cynical bone in their collective body. As they play, every note and phrase screams "belief!"

Just as The Earlies - another unfeasibly large band - have managed to breathe new life into the corpse of epic rock, so Jaga invest the disputed musical territory between left-field prog and jazz with a sense of absolute conviction. As Jaga have been doing this for 10 years, they're getting very good at it, too. When I saw them for the first time at The Big Chill festival two years ago, it was merely awe-inspiring.

Musically, where does one start? Perhaps with the dizzying doubling of instruments: Mathias Eick, whose trumpet solo against the onrushing wave of an ensemble chorus, produced one of the best and most conventionally jazzy moments of the afternoon, also covers double bass, vibes, vocals, percussion and keyboards. Lars Horntveth moves from slide guitar to bass clarinet, sax and keyboards. His sister Line parps a tuba, shakes percussion and sings wordlessly; on drums, brother Martin is Elvin Jones meets Animal from The Muppets - his primal energy epitomises the band's commitment.

Add another six, equally essential, members covering everything from Mellotron to glockenspiel while also operating as razor-sharp rhythm and horn sections and you're left with quite a band, although it still came as a surprise when they all downed instruments to sing 10-part harmony. The new album What We Must tells part of the story, but you've got to hear Jaga live.

It's not often you get to see a real whirling dervish, especially in Cheltenham. The dancer Mira Burke, who appeared with the incredible band of Turkish/Canadian DJ Mercan Dede, revolves with unerring grace at increasingly dangerous velocity to the Sufi-trance music's blend of electronics and old-time instruments. Mercan Dede (his name means "Coral Grandfather") cues up subtle beats and samples while four Turkish traditional musicians play often wild, sometimes Gypsy and Balkan-inspired, modes that bring ancient and modern together. The young clarinettist, Huseyin Ceylan, reached mystical heights of invention: was he playing the horn, or the horn him? An object lesson in how world music can function without tying itself up in knots about "authenticity" or "fusion".

This was an under-attended but excellently programmed and appointed festival that next year - and there will be a next year - should become an essential part of the seasonal calendar.