Steve Earle and Shawn Colvin, Union Chapel

The Union Chapel is packed – upstairs and down, and the two of them emerge through a backstage curtain, step up to the mics and sing The Everly Brother’s "Wake Up Little Suzie", which should hit the spot just right, with the sweet-n-sour harmonizing of their voices, Shawn Colvin’s picking technique against Steve Earle’s rhythmic sense behind a guitar, but here and on a few songs through the set, it doesn’t quite get there.

At times their guitar playing doesn’t fuse, at times it falters, as if they have quite settled yet, their voices pull against each other rather than together. I notice that Earle (I suspect he could be voluble in his  sleep) sometimes talks over her – when he sets about introducing the encore’s closing song, "Copperhead Road" [huge cheer], by saying, “Okay, I’m taking over here” I sense that Colvin only half-jovially covers her face with her hands. 

While there are fine moments, it feels like they could be tighter

Second song "Come What May" is a lot stronger, cowritten by Colvin and Earle and one of several upbeat, mid-tempo gems from the duo’s eponymous album on Fantasy, recorded by Nashville producer Buddy Miller at his home studio. As they first shared a stage together in 1987, this set is a long time coming, and it’s a fine album, with Miller contributing excellent guitar as part of a six-piece band. The band’s not here, alas. This performance is billed as “as intimate evening”, so it’s down to the two of them to carry it off, and not on a stretcher. And while there are fine moments, for sure, it feels like they could be tighter, and stronger.

Of the highlights, Sylvia Fricker’s beautiful "You Were On My Mind" is one of a number of strong covers, while the Stones’ "Ruby Tuesday" is taken stately and slow, a Southern carriage driven through a perfect example of Sixties British psych-pop. The Beatles’ "Baby’s in Black" is a sweet country lament, while the lean electric riff on The Nashville Teens' "Tobacco Road" translates less well. Nevertheless, Earle’s electric Gilchrist mandolin gets the first of several adventurous outings here – best on the powerful closer, "You’re Still Gone", which Earle dedicates with feeling to the late, great Guy Clark, whose ashes, he reports, had recently been taken to their New Mexico resting place.

Elsewhere, Earle’s introduction to the beautiful "Someday", a song he wrote in 1985 and Colvin covered a decade later, does ramble on but along with "You’re Still Gone", it is the highlight of the set. And while I may have had reservations, the paying crowd – it’s just shy of £40 a pop – absolutely love it, and the standing ovations before and after the encores are long and rapturous.

About an hour later, the Chapel closed, and the terrace it’s on quiet and deserted, I’m passing by with a friend towards the Tube when we pass a striking blond woman in dark glasses sitting on a doorstep, another woman standing beside her. It’s Shawn Colvin. We stop and thank her for the gig and she thanks us back, looking a little weary, and the direction of her gaze falls back to the cigarette in her fingers, burnt about halfway down. “I write break-up songs, that’s about it,” she’d said, introducing her 2006 song, "Don’t Worry Me Now". You hope this musical relationship is one that develops and lasts the course.