Following Stereolab's break-up in 2009, Sadier began a solo career that has so far produced two albums, and when we speak, the Parisian singer is in her adopted home, London, preparing to put the finishing touches to the third. As with her old band, the solo records couch some quite radical ideas in music that is full of sweet melodies and Gallic style.
"In my album I tackle the fact that we are at war, now," she says. "People don't know it, you see. The ultra-rich know they're waging a war. They are very aware of it and very organised and they are winning. In the meantime, people don't even realise that there's a war. They are so oblivious. We're so manipulated and stressed out – depression, anti-depressants, the new things to buy, television; everything is sedating us and stressing us out at the same time. I tackle those themes in my songs. Among other things."
Sadier talks passionately and at length along these lines, railing at the financial system and the government's failure to regulate it, and at the left's failure to provide an alternative. But, she tells me, she is an idealist at heart. "We have a lot of potential as human beings, and individual human beings have a role," she says. "This is a song I'm working on now – each and every one of us has something unique to offer. We are only in the universe once, ever, and therefore we have one very unique role to play in our lives. The idea is to find that and honour it. It's a beautiful, amazing journey.
"We are living in a neoliberal, global capitalist system that is not making the best of people. It's pushing them towards the worst version of themselves. But very cleverly – by people who know exactly what they're doing. I think the system plays a lot on those things, and that's what is holding people back."
Despite these times of austerity, it is unusual to find a musician so willing to talk at length about politics and economics, but then these are themes that have always punctuated Sadier's career – in the band McCarthy where she met her former musical and romantic partner Tim Gane, in Stereolab and now as a solo artist.
"I'm surprised there aren't more people out there screaming their hearts out," she says. "To me, the role of an artist is to make people feel something; make them react with their brains, with their nervous system, with their senses, one way or another.
"And to resonate with something really true, rather than covering the truth up with s*** and turning it ultra-kitsch. I am not a cynic and I do not like cynical art. It doesn't do anything for me."
And yet, for all that she talks about 'screaming', her scream is metaphorical. Sadier's music draws on French chansons, psychedelia and classic pop, and sounds very different to stereotypical 'protest music' styles like punk rock, hip-hop and raw, angry folk.
I suggest that perhaps there is some subversion at play: "You're right, maybe I'm really being subversive!" she laughs.
"My boyfriend bought a Marcos Valle record today, and in fact it was all rebellious stuff. It was a dictatorship [in 1970s Brazil]. And I tell you, it's not the first thing you pick up when you listen to those records, which are incredibly beautiful.
"I'm not sure where people get the idea that if you're angry you should play angry music. No! There's no rule. That's what I like about art – you make your own rules. That's really lovely. When I see someone's art, I want to see or hear the person, not some formal idea of what a painting or a song should be. F*** that, that's boring."
Sadier's upcoming gig in Belfast is a support slot for Neutral Milk Hotel, a revered and much-loved band who recently reformed after a long period of inactivity.
Of course, this begs the question of whether Stereolab might one day return. They called it a 'hiatus', after all; while Sadier and Gane are on sufficiently good terms these days that he contributed to her second solo album. Sadier gives little away, saying only, "It could happen, but it's not up to me."
Given that she and Tim Gane split up as a couple over a decade ago (they never married but have a teenage son), perhaps it's surprising that the band continued – with both of them as key members – for as long as they did. Sadier paints a picture of a band full of disquiet and resentment.
"It was sour in the band, you know, which is really a shame because musically it was so good," she says. "In the band we never got it right, like 'Cool, we're happy to be together'. Maybe it's just being young and egotistical and stupid. But also, some people are made for the road. They are nomads. But some people are fundamentally sedentary. They should stay home and not tour, because it makes them deeply unhappy, and that unhappiness will reflect onto others and that's going to rot your band. I'm a nomad, but the others, some weren't."
That wanderlust has taken her to far-flung places in her solo career. As soon as Stereolab broke up, she received offers to play gigs all over the world. "I could barely play the guitar and I just went off playing Greece, Portugal, Belgium, Spain, Brazil, Chile, Argentina..." she says with wonder.
But despite her French roots, London is where Sadier feels at home. "I'm a Londoner," she says firmly. "I've spent more time in my life away from France than in France. I'm not tempted to go back to France – not in the next 20 years. Maybe when I'm 60 or 70! France is not for me; I don't fit there. I really dig London, actually. It works for me."
So why would a Frenchwoman feel so ill at ease in France? "People are depressed," she says. "There's not much freedom in the law or in people's mentality. I wanted to form a band [in the 1980s] and people called me crazy, but in London it's no problem – get some amps and let's go and rehearse. 'A gig? No big deal, there are thousands of gigs every day'."
She exaggerates her already strong French accent: "'Un concert?!' – it's like a highly sacred thing. It's easier to do things here, or certainly in the late 80s when I arrived. Also, you're not judged constantly, or at least not in your face. Maybe people judge you and keep it to themselves, or talk about it privately at night. You're more accepted as you are here."
All in all, Sadier seems at peace with how things have panned out – in life, in Stereolab and in her still relatively new career as a solo artist.
"For me it's very exciting because finally I get to do what I love doing," she says.
"I was frustrated with Stereolab because my hands were cuffed behind my back, and all I could do were the things Tim couldn't do. I accepted that and gladly did it, and I am very thankful, but still there was a very frustrated part of me that wanted to write music. I dreamt about songs. I wrote them in my dreams. And I didn't have the confidence that I could write. But I would still do it and it would be like a miracle each time.
"I realised that I could write songs when I wanted to do it, and that they are good and have value. I'm really pleased with the journey I've been on."
Laetitia Sadier plays The Limelight 1, Belfast, next Thursday, May 15 in support of Neutral Milk Hotel. For details, visit www.limelightbelfast.com