Anyone who heard Mitchell's folk opera Hadestown at the festival two years ago will understand Polwart's choice. With a voice both sweet and rugged, with guitar licks bluesy, bold and feminine, with deep eyes gazing out behind flaxen locks, Mitchell is a seductive troubadour indeed. Mostly she sings high and girlish, throwing her head back at the corners of phrases with a fresh country lilt. But it's her low register, soft and earthy, that really captivates: I wish she'd use it more. Here she and guitarist/singer Jefferson Hamer (more than a touch of James Taylor to his voice) performed their new album Child Ballads, which taps into the 19th century collection by Francis Child. Many are familiar from versions by the likes of Martin Carthy or Nic Jones, both of whom Mitchell name-checked as great influences. But she and Hamer make these songs their own, weaving close-harmony Americana around timeless English and Scottish material. Lau accordionist Martin Green and fiddler Aidan O'Rourke added stylish textures in a couple numbers. The disc is out next week; highly recommended.