2012’s beautiful I’m A Dreamer was a charming collection of gentle crooners, creaky and lived in, blending loose jazz arrangements with folk song writing. It was half surprising that you couldn’t hear the crackle and hiss of an old gramophone needle.

Sat in front of us in concert, she gives an even stronger sense of having wandered through a wormhole. Performing solo, her songs are an even more delicate blend of old time Americana folk, with subtle echoes of music hall and a voice that gestures towards her early-career operatic ambitions.

Even her more incidental details – from her drawling Colarado speaking voice (all “gollys” and shy mumbles away from the mic) to her large, grandmotherly glasses frames – combine to strengthen the eerie impression of having slipped into the 1930s.

Tonight’s show continues a similarly stripped back approach to this year’s collection No More Lamps In The Morning, which saw her excavate and reshape old material in looser, sparser arrangements.

Tonight (25 February), the first half of her set is performed entirely alone, accompanied by a modest electric guitar with soft reverb. On record, some of her songs have a little rhythm, a little swing. This evening, her songs are fragile and sober, melancholic in the main – especially the stunning numbers she performs for piano. This further strengthens the feeling of time travel, especially among the flickering candlelight of London's Café Oto.

Halfway through the set, Foster diversifies her sound, and proves that her appeal is based on more than creating a well sustained mood piece. Her songs gain a lot from being accompanied by the dance of melodic and lively violin for a few numbers.

Towards the end we enjoy an even bolder gambit: the introduction of heavily treated electric guitar, bass turned up high, random slide runs battling upstream against the delicate piano chords. Fantastically, the performance more than withstands the stylistic rupture. Indeed, it thrives all the more.

Foster is so much more than a sophisticated impressions act. The mood she wraps the room is a powerful exercise in nostalgia, going further into the past than most of her contemporaries, not to mention executing it more skilfully. But her songs are never reduced to just being vehicles for this old-world conjuring trick. They are warm lullabies that work on their own terms, and lead us through an hour of song writing in one of its most inviting and comforting forms.