Roy Harper arrives on stage at the Festival Hall with a healthy selection of excuses. He hasn’t played in three years. He’s only had half a soundcheck. He met the soundman at four o’clock – no, at ten past five. The first song is brought to a temporary halt after about thirty seconds, due to his guitar sliding on the passport secreted in his trouser pocket.

Roy Harper arrives on stage at the Festival Hall with a healthy selection of excuses. He hasn’t played in three years. He’s only had half a soundcheck. He met the soundman at four o’clock – no, at ten past five. The first song is brought to a temporary halt after about thirty seconds, due to his guitar sliding on the passport secreted in his trouser pocket.

Interestingly, though, none of this appears to faze or frustrate Harper a great deal. And in fact, talking in the office this morning, it seems as if his shows were ever thus; ambulatory, digressive, illuminated by miraculous moments. Lured out of a notional retirement at the behest of Joanna Newsom – “One of the most beautiful women I’ve ever seen,” says Harper, perhaps over-keenly, after having earlier admitted to a “bohemian” love life – his solo set tonight is a small, eccentrically-shaped gem.

Retirement and glitches notwithstanding, Harper’s guitar playing and singing are as unsteadily heroic as ever. “Me And My Woman”, “One Man Rock And Roll Band” and “Another Day” remain uncannily potent, though more recent songs like “Pinches Of Salt” and “The Green Man” measure up well, too. It’s all scattily compelling, and also makes me rather ashamed that my knowledge of his work is fairly limited.

No such limited knowledge of Joanna Newsom, which accounts for a certain amount of fanboy grumpiness about tonight’s 12-song setlist.: no “Emily”, “Sadie”, “Colleen”, “Cosmia”, “Bridges And Balloons”, “Only Skin”, as well as at least two missing favourites from “Have One On Me”, “In California” and “You And Me, Bess”.

Of course, there’s a lot more of “Have One On Me” to concentrate on and, a solo opener of “’81” apart, much of the selections appear to have been made to best show off the skills of Newsom’s curious and excellent band, consisting as it does of two violinists, a trombonist, multi-instrumentalist and arranger Ryan Francesconi (who flits between Bulgarian tambura, banjo, guitar and recorder), and drummer Neal Morgan.

Morgan doesn’t so much provide punctuation as add idiosyncratic rustle and flurry to Newsom’s songs and, as he discreetly gropes and raps his cymbal during “The Book Of Right-On”, it’s odd that, in contrast to previous Newsom shows I’ve seen (mostly solo, but also with a full orchestra circa “Ys”), this one feels like much more of a band production, with the intense focus shifted a little away from her.

That idea’s compounded by a treatment of “Inflammatory Writ”, a sketchy piano pounder on “The Milk-Eyed Mender”, now grown into an ornately jazzy full-band piece. It’s a pleasant shock, too, to hear how these old songs lose none of their charm when sung by Newsom in her new, post-nodes voice: if someone had been introduced to her music at this show, it’s hard to imagine them being aware of the alleged vocal awkwardness that enraged her detractors for so long.

Her songs, though, are still wilful and unorthodox beasts, none more so than a sensational “Have One On Me”, played second in the set, as if to warm up the band by putting them through one of their toughest assignments early on. The final movement’s resolution, when Newsom is joined on wordless harmonies by the two female violinists, is one of the highlights of an exceptional night.

Another is “Kingfisher”, played after a lengthy tuning break-cum-question and answer session, during which Newsom (and Morgan, a kind of straight man in an unlikely double act) proves emphatically that the image of her as some permanently precious, humourless, cloistered sprite is wide of the mark. Francesconi gets to play a selection of recorders, and the intricate arrangements – which very occasionally clutter and obscure Newsom’s power and songcraft – work beautifully.

“Good Intentions Paving Company”, perhaps understandably, gets the biggest cheer of the night. But again, it’s some of the songs from “Have One On Me” that I’ve slept on, comparatively speaking, that really shine: “Autumn”, “Occident”. A bit more proof, maybe, of the strengths of that album. And a great gig; if you were there, or if you’ve seen any other dates on the tour, let us know what you thought.