"I'm going to channel my inner Belinda Carlisle for you," Thea Gilmore promises, and pauses half a beat. "But without such good bone structure." Gilmore excels at self-deprecation, but the song that follows, Start As We Mean to Go On – which she wrongly describes as "a powerpop anthem" ( it's actually closer to Natalie Imbruglia's wistful Torn ) – packs an emotional wallop that renders bone structure irrelevant.

Fourteen albums into her career, the Oxford-born songwriter has settled into a best-kept-secret role that allows considerable freedom. She makes exactly the records she fancies, such as 2011's Don't Stop Singing, which paired her musical arrangements with a collection of unrecorded Sandy Denny lyrics; and she's able to change her live show at will – at the tiny Ropetackle Centre, she's taken a break from her usual 10-piece set-up and gone acoustic, backed by a violinist, a cellist and her guitarist husband, Nigel Stonier ("It was either death or marriage," she says, and he looks suitably flattered).

Gilmore tells us she writes "a lot of songs" – so many that the merchandise table offers an EP of tracks that wouldn't fit on to her current album, Regardless. The 15 she plays tonight serve as a reminder of the breadth of her talent. Singing about her own life, as on You're the Radio and I Will Not Disappoint You – a hymn to her young children – she's breathy, drifty and very English; when she looks outward, her warm alto sharpens, and she's as insistent as Billy Bragg. The Amazing Floating Man, aimed at the banking industry, is performed a cappella, making every cutting word pin-sharp; Beautiful Hopeful acerbically advises aspiring singers to "play it pretty like a good girl should". A crystalline version of Denny's Pain In My Heart is the highlight: if Denny had been able to pass on the baton, Gilmore might well have been her choice.

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