There's a really nice session in the Golden Ball in York every sunday night from about 8:30. Its intended to be American old time music, but it varies a bit depending on who's there - usually 2-4 fiddles, 2 mandolins, 1 banjo (if I'm there), a couple of squeezeboxes, 1-2 guitars and a bass. It's near the city wall just south of Micklegate, but I can never remember the street name! Excellent real ale too.

Cheers
Ray