Karine Polwart, Roxy Art House, Edinburgh

If ever there was a classic case of artist and audience meeting on terribly comfortable ground, Karine Polwart's performance at last night’s fundraiser for the Green Party was it. Held in a beautiful converted church, there was more than a trace of the Vicar of Dibley lurking around the edge of the proceedings.

A gifted singer and composer with a strong pedigree in Scottish traditional music – she has served time with stalwarts like Malinky and the Battlefield Band – in recent years Polwart has stretched her wings into something less easily defined. It’s hard to place her. She’s not really folk and she’s not quite your average singer-songwriter. Polwart makes soul music for a nation raised on Calvinism; political songs with a deeply personal imperative. She studied philosophy at university and later worked for Woman’s Aid, and in her music there remains a clear determination to be socially useful.

Polwart had to be on her game last night to stop support act Ewan McLennan stealing the show. The Edinburgh-born, Leeds-based young folkie was a new name to me, but proved himself a fine singer – holding the crowd rapt with two unaccompanied vocal pieces – and a dexterous guitarist, using open tuning to create a vivid, fluid sound. Every song came with a story, often longer than the song itself, which ensured he ran over his allotted time.

Drifting up into the furthest reaches of the high vaulted church, this sounded like a different kind of sacred music

No one seemed to mind much, least of all Polwart, who humphed on afterwards with indecent haste. This was her last solo performance before she headed off on extended maternity leave, a point she emphasised “just in case you thought I’d been eating too many pies lately”. She will be missed in her absence. Polwart has the purest of voices and a gift for words that cuts beyond the often polite veneer of the music. Accompanied with unostentatious dexterity by her brother Steven Polwart on guitar and vocals, and by the impish Inge Thomson, who accomplished the eye-popping feat of playing the accordion and the cymbals at the same time, while also singing pitch-perfect harmonies. There was a running gag about a green dress, first worn by Polwart last year “as a party political broadcast”, and last night worn by Thomson. It’s apparently destined for Mr Polwart’s next outing.

If this was all very cosy and occasionally teetered on the edge of twee, it was balanced by the hard truths of Polwart’s songs. Her 10-song set, clearly truncated because the evening had over-run, mined her last album, 2008’s This Earthly Spell, and its predecessor, Scribbled in Chalk. "Better Things" was a deceptively gentle diatribe, a spirited arm-wrestle between the hands that made Trident (“A bullet in a bully’s pocket”) and the “hands that pull a baby free”.

Appropriately, there were several references to children in these songs. "Rivers Run" was a love note both to the earth and her young son, but it was comprehensively bested by a new song – which appeared to be called (and I'm going for a strictly phonetic translation here) "Bee-o, Bee-o" – about rushing back to the boy “waiting at the window”. Drifting up into the furthest reaches of the high vaulted church, this sounded like a different kind of sacred music, intended to glory in everything living rather than all that has passed away. As if to emphasise the point she also sang "Sorry", an attack on those who have wreaked havoc in the name of religion and then “nail [their] remorse to the cross”.

Pregnancy clearly hasn't slowed Polwart down. She is part of the sprawling collaborative project Burns Unit and has been working on an EP with Lau, the results of which are due soon. She also contributed to last year’s Darwin Song Project and played two of the songs here, the bubbling "Trust in the Rolling Ocean" and the very moving "We’re all Leaving", inspired by the death at an early age of one of Darwin’s daughters. 

She ended with "The Good Years", her collaboration with Scottish poet Edwin Morgan, and another paragon of the kind of melancholic optimism in which Polwart seems to specialise. She came back for a brief encore of "Follow the Heron" and then called it a day, mindful that it was getting late (all of 11pm) and no doubt aware that a substantial part of the audience were starting to look at their watches and mutter about babysitters. It was that kind of night. Chalk it up as a high scoring draw between the heartfelt, unguarded music and the sensational vegetable curry pasties.

• Find more information on Karine Polwart