Near the middle of their set, Northumbrian sisters Rachel and Becky Unthank step away from their microphones to sing unamplified, taking advantage of the airy acoustics of the semi-sacred space of Brighton's St.George's Church. The tune is "John Dead", a sea song "collected" -- as they say in folk circles -- from St Vincent in the Windward Isles. It was handed down to the Unthanks by shanty singers the Keelers, who count Rachel and Becky's father, George, in their number.