Veteran guitarist Martin Carthy and his fiddle-playing daughter Eliza are here to play from their new album, a spartan duo collection called The Moral of the Elephant. Despite working on nearly 200 LPs between them, it's their first as a duo, and it's a welcome chance to hear them unfettered by the ugly clamour that a rhythm section can often impose upon their work.

Tonight's show reminds us that folk music, at its best, doesn't groove like rock, blues or jazz music. Its pulse is dictated by the narrative: it pauses, it hesitates, it bends time. Such elasticity suits Carthy senior's extraordinary guitar style. Using his own arcane tuning - CGCDGA - he turns his instrument into a baroque harp, his left hand a tangle of spidery fingers and wrapped-over thumbs. He'll often suspend time by lingering over some of his weirder chords, like a medieval lute player who has surprised himself by unwittingly inventing bebop. When the Carthys are joined by sitar player Sheema Mukherjee for the first third of this show, it reveals how much Martin's guitar style has in common with a sitar: the same drones, dissonances and filigree flourishes.

Eliza here takes the fiddler's role that Dave Swarbrick has long played with her father, but she's a more agile player, weaving in and out of Martin's melodies using complex harmonies and counterpoints. She also switches effortlessly between voice and violin, and has clearly recovered from the nodule damage that has plagued her in the past. Her voice is amazing on Happiness, a charming parlour ballad written by Nick Drake's mother, Molly Drake. During the song's outro, the Carthys are joined by Nick's sister, the actress Gabrielle Drake, who reads one of her mother's poems. It's a poignant reminder of another parent/child musical dynasty who have quietly revolutionised traditional English music.