En route to meet Annie Lennox, I am beset by a trio of Annie Lennoxes. The poster campaign for the Eurythmics' new Ultimate Collection album features iconic images from the band's 1980s heyday, when Lennox, one of the first MTV superstars, changed personae as frequently as most people changed their socks. So here's the tangerine-haired androgyne of Love Is a Stranger, the coral-clad mime of Here Comes the Rain Again and the Marilyn-wigged siren of Thorn in My Side. After seeing the video to Love Is a Stranger, one aghast US critic branded her "a youth-corrupting transvestite".

Today, inside a forebodingly grand London private member's club, the real Annie Lennox is a trim 50-year-old, her youth-corrupting days far behind her. She walks in and immediately lies down on an antique sofa, as if transforming it into a psychiatrist's couch, but her prim, precise speech sends a different message. She thinks Ultimate Collection, which includes two new songs and coincides with a handsome box-set of the duo's eight albums, is "appropriate", a favourite term of approval. Even when she's discussing her "intrinsic self" or the miracle of music-making, older, sterner concepts of being "upright" and "respectful" keep poking through like bits of wire.

Today she is shouldering the promotional load alone because Dave Stewart has moved to Los Angeles. Before they reformed in 1999, the Eurythmics' career (eight albums, 30m sales) almost perfectly bookended the 1980s. In the Garden, their neglected 1981 debut, was mysterious avant-pop, featuring members of German cult group Can; Sweet Dreams (Are Made of This), their commercial breakthrough, was a synth-pop colossus, still sampled by the likes of MIA. But, by the time they split in 1989, arena tours and saxophone solos had buffed away their rough edges, not to mention their friendship: they didn't speak for three years.

Lennox likes "tying up loose ends" but doesn't much care for analysing the Eurythmics' legacy. Did she listen to the box-set? "Once. I was asked to. But I wouldn't choose to. There's something slightly creepy about that. You'd have to go to a secret room somewhere to do that."

These days, Lennox works at her own pace; her 2003 solo album, Bare, was only her third and she only tours when she wants to. In the past two years, she has exclusively played benefit gigs: for tsunami relief, Nelson Mandela's 46664 campaign and Live8, where she used footage from her recent Comic Relief visit to HIV-positive children in Uganda.

Does she think Live8 worked? "Yes," she says immediately. "It's a pity that we couldn't have raised money but that's not a criticism. It succeeded as a platform for the issues. Nowadays people's attention is like fashion. It comes in - yesyesyes - and then five seconds later: next!"

Lennox grew up in an atmosphere of political debate; her father, who worked on the Aberdeen shipyards, was a socialist. "There was this whole working-class ethos of communism and trade unionism. It came from the shipyards. Unfortunately they didn't know what was really going on in the Soviet Union." The thought seems to sadden her, and she switches tack. "I'm not a party political person. I want to be a grass-roots activist from outside the parties because I don't trust them."

To that end, she works with the likes of Amnesty International and Greenpeace, but she is wary of proselytising. "You have to be very, very careful about it - so careful - because you can misrepresent the issues if you don't fully understand them. Also, people don't want to be preached at. It's not appropriate from a place of wealth and fame to start telling other people they should do A, B or C. I think it's a turn-off."

For a decade or so, Lennox seemed to be ubiquitous. She won the Brit for best female artist so many times (six) that it might as well have been renamed the Annie Lennox award. Her first solo album, 1992's Diva, was a multi-platinum success. But then she began her retreat. Promoting Medusa, her 1995 collection of cover versions, she played just two dates, and then took a four-year sabbatical. The reasons were manifold: writer's block; raising her two daughters, Lola and Tali; and the desire for a holiday from fame.

"I had this space to not have a schedule, to not be dictated to by success, to just become ... a human being," she says. "I wanted to be able to go to the creche and be a mother with all the other mothers. That's the weird thing about fame. At the time maybe the idea is, 'Oh I have to be something special and above everyone else,' but at the end of the day you just want to be incorporated with the rest of mankind." Whenever she feels herself complaining too much about press attention or the pressures of touring, Lennox brusquely checks herself. She has seen worse ways to earn a living. "My parents were post war. They didn't have much fun. There was a lot of hardship and a sense of lack and a lot of fear. And then the Scottish stoicism of not being hugely expressive in a tactile fashion. I wanted to be expressive with my kids."

Lennox was always conscious of being an only child and an outsider, which explains why she bonded so intensely with Dave Stewart. They met in 1976, when he walked into the vegetarian cafe where she was waitressing and asked, jokingly, "Will you marry me?" She had moved to London to study flute at the Royal Academy - "Flute!," she exclaims, as if nothing could be more absurd. "I spent three years there trying to figure out what I wanted to do before I went back to Scotland with my tail between my legs. Fortunately, I met Dave and felt in him a kindred spirit."

They dated for four years but their creative partnership proved much more resilient. The garrulous, hyperactive Stewart liberated Lennox from her sometimes paralysing perfectionism. "Wherever he is, whoever he's with, he's not fazed by anything at all," she marvels. "He's fearless in that sense. And when it comes to music-making he's a facilitator." On her own, she says, she sometimes finds it hard even to walk up to a keyboard and start writing. Before almost every gig - although not, strangely, Live8 - her heart pounds and her palms become clammy.

If she hadn't met Stewart, could she have been a solo artist? "No. Nooo. I didn't have the drive. I didn't have the self-belief. I didn't even think I wanted to do that until 1990. I was actually content to be defined as half of a duo."

Did she surrender too much of her identity to the Eurythmics? "Possibly. I really divested my own independent, individual self into being one half of a duo. In fact I didn't often say 'I'. I would quite purposely say 'we'. It seemed appropriate."

In her private life, Lennox is currently "I". Bare explored the fallout from her divorce from her second husband, Israeli film-maker Uri Fruchtmann, and did nothing to contradict the abiding cliche of "tragic Annie". She's had her share of challenges - a first marriage that lasted less than a year, a stillborn child in 1988, lifelong experience of depression - but I wonder if she recognises the portrait the Daily Mail painted, around the time of Bare, of "Britain's most tortured rock star" and "the despair that has dogged every step of her life". "It's not great because it's not really the truth," says Lennox, with a sudden cold snap in her voice. "There are certain things that have happened in my life that have been difficult. But am I tragic? I don't think I am. I think I'm quite noble actually. I think I'm a warrior, to be frank. I get up in the morning and face the day, whatever it takes."

In 1998, she described herself as "a huge success at ordinary life". Would she still say that? "Hm. I can see why I said that at the time and I don't know if I would say a huge success now." She searches for just the right formulation and smiles when she finds it. "I would hope I've been a bit of an elegant survivor."

· The Ultimate Collection and Eurythmics Boxed are both out now on Sony BMG