“We hope you like banjo!” Since there were five of them on stage, and no other instrument in sight, Abigail Washburn’s opening salvo seemed reasonable. Washburn and her husband Béla Fleck are banjo royalty – he redefined bluegrass with the Flecktones, she fused the sounds of Appalachia with Chinese folk – but this year is the first the couple have recorded together. Imagine Posh and Becks launching a football kit and you’ll understand the excitement of their fans.

For some, a banjo duo might sound like answer to a joke. What’s worse than listening to a banjo? Listening to two! But from the moment the pair opened this London jazz festival set with Railroad, their bluesy version of the folk standard, the extraordinary expression of a much-maligned instrument became apparent. In the duo’s hands, their combined total of 10 strings can span the range of a piano, a blues band, or an entire symphony orchestra.

Their harmonic lines were as close-knit as their relationship, and there was warmth and wit woven through their performances, smiles and patter. As Fleck reached for his piccolo banjo, to accompany the soulful Ride to You, he asked: “Honey, does this banjo make me look fat?” But it’s their expressive range – in Washburn’s nostalgia-tinged voice and Fleck’s insouciant solos – that made the evening so compelling.

With so many genres at their fingertips, one danger might be over-flavouring an already rich soup. Instead, each piece is a crystalline distillation of emotion. When Washburn flips the murder ballad on its head in Shotgun Blues – “It’s always the girl that dies,” she complains – she has the crowd in her hand, and, finishing with a gospel song she learnt at Doc Watson’s funeral, she has them on their feet.

If anyone can convince a sceptical world of the beauty of the banjo, it is this pair.

• The London jazz festival runs until 22 November.