She fell from public grace with the end of her marriage, but Kate Winslet isn’t down at all. As Iris opens, with a terrific performance by her, she talks candidly to Garth Pearce about why she was right and the tabloids were wrong.

Kate Winslet was one of our own. She reveled in her womanly figure, ate bangers and mash at her wedding reception, gave birth to a daughter, Mia, now 15 months old, and spurned Hollywood. Then came the shock announcement that her three-year marriage was over. Even worse, it seemed that her nice, homely husband, Jim Threapleton, was being swapped for the high-flying director Sam Mendes. She found herself being ridiculed on television by Mick Jagger and attacked in a campaign by a national newspaper. The fairy tale seemed over.

But Winslet, fresh from divorce last month for her husband’s "unreasonable behaviour", was putting on more than a brave face when we met last week. Now 26, she is aware that the attitude towards her has soured, knows that the reassuring normality of her life has been shattered. She has watched the news, heard all the references to her on daytime television, read the newspapers. "I am not going to do some massive cover-up on how I really feel," she says. "I do genuinely feel great."

She looks it, too. It’s a dismal winter’s morning in London, but her black, figure-hugging trousers and high-necked top, leaving her arms and shoulders bare, would grace any cocktail party. There is no sign of the effects of an alleged crash diet. She looks healthy, glowing and sexy, rather than thin or strained. Her blonde, shoulder-length hair is parted in the middle, and her big blue eyes are steady, if a little wary. A slight furrow in the brow is the only sign of recent stresses.

"It would have been nice to play out the past few months of my life in private, as opposed to being on the front pages of most of the newspapers," she says. "But it is one of the things that comes with being known. It sometimes makes me irritated, and occasionally angry. I have a mechanism for looking on the bright side, and I try to be good at accepting things. I have seen headlines and have been quoted about my life and marriage when, honestly, I have not spoken myself until now. Apart from anything else, I did not feel like speaking to anyone."

The meeting is to discuss her role in the film Iris, directed by Richard Eyre, in which she plays the young bohemian writer Iris Murdoch, alongside her devoted husband, the university lecturer and literary critic John Bayley (Hugh Bonneville). It is another superb performance from Winslet, who, for my money, is the world’s most talented twentysomething actress, and has garnered her a Golden Globe nomination for best supporting actress. But more of this later. To ignore the recent off-screen dramas of her own life - and her desire to set the record straight, so far as she is willing and able - would be like ignoring all the courses of a dinner and going straight on to dessert.

The announcement of her marriage break-up, in September, was even more surprising than that of Tom Cruise and Nicole Kidman earlier in the year. "There has been emotional turmoil in my life," she agrees. "There is no getting away from that. I had to deal with it as best I could. But my main priority is Mia, so there is an instinctive strength kicking in with her. She was the priority for both Jim and me. She is a fantastic kid and a source of tremendous joy."

When Winslet finally knew her marriage was over, who was the first person she told? "Jim," she says. "He and I were very honest with each other, so it meant that communication never stopped, and nothing was distorted. It was not working, and had not been working for some time. I am afraid that it is as black and white as that. Whatever has been said since, the parting was mutual and continues to be amicable.

"I know, from the private reaction I’ve had, that it came as a big shock to people. It was easy to speculate that we were young and had simply given up. But we had known that things were not right, and we were trying to make it okay. We were not giving up on a marriage easily. But there is no point in simply going through the motions. It was not easy and not pleasant. At the end of the day, you cannot put a square peg into a round hole."

She is not prepared to spell out details, and is obviously reluctant to discuss such things at all. She also stresses - possibly as many as a dozen times in our talk - that baby Mia comes first, second and third in her life. But, although she politely refuses to answer on several points, there is no hesitation in what she does say. "I am the same person as I’ve always been, in that what you see is what you get," she says. "I am who I am, and there’s no getting away from that. So, when I sit here and say it was hard, that’s true. But it was also absolutely the right thing to do, and that is what I really mean. It was incredibly frustrating when people tried to speculate about some deep-seated reason as to why it all went wrong."

There was also no overlap between her marriage ending and the start of her relationship with fellow-Reading-born Mendes, the 36-year-old Oscar-winning director of American Beauty. That is absolute fact. She is also determined not to speak about him. "All I would say, without talking about Sam, is that being a parent is my priority," she says. "It meant a lot of time at home last year, when I would watch morning-television discussion programmes. I would hear that Jim was left holding the baby and I was working all the time.

"Well, number one, if one parent is away, why shouldn’t the other parent be holding the baby? It is better than a nanny. And, number two, I only worked for 11 weeks last year, and even had her with me during those times. I had decided to do that with work even before she was born. I am lucky that, as an actress, I can take my baby with me."

She spent six weeks in Austin, Texas, at the end of the year, on a new film by the director Alan Parker, The Life of David Gale, playing - of all things, at this particular moment - a journalist. She is requested by Gale, played by Kevin Spacey, to write his life story just three days before his execution for murder. "She gets very involved with his life, and starts to question whether he is innocent or guilty," she reports. "It is not a true story - but it is contemporary. So to play a character who runs around in jeans, for once, is great.

"And I loved working with Alan Parker. He decided that we were related in a past life. We started to anticipate what the other was feeling or thinking. I would go to him with a script and say: 'You can tell me where to go, but I have had a couple of thoughts on this.' And he would reply: 'I know what you are going to say.' He would then practically read my mind and tell me. He was always right, too. We had the greatest communication, and it was great, for me, to be working. I feel that I am flying and really buzzing when I’m working."

It shows in Iris. She plays a woman who was ahead of her times: quizzical, idiosyncratic, bisexual. Judi Dench plays the older Murdoch, with Jim Broadbent as her husband, in a moving story of how their enduring love survived even her Alzheimer’s disease, which eventually killed her, at 80, in 1999, three years after the publication of the last of her 25 novels. There are Golden Globe nominations, too, for Dench, as best actress, and Broadbent, as best supporting actor. Iris deserves the accolades: it’s a gem of a film with immaculate performances.

This is Winslet’s first on-screen performance since the insecure plump girl in specs, working on the top-secret Nazi Enigma machine at Bletchley Park, Buckinghamshire, in the elegantly filmed second world war drama Enigma. She filmed in April 2000, during her early pregnancy, when, presumably, all was well at home. By the time the film had its London premiere, four months ago, she was embroiled in divorce proceedings and could not attend. Mick Jagger, the film’s producer and the subject of a lively television documentary, Being Mick, mocked Winslet’s apologetic statement by reading it to the cameras and mimicking her voice.

If she took any offence, there is not even the slightest sign. "That is just Mick, and he impersonates people," she says. "He is very funny. It was easy to ask why, as a superstar, did he want to produce films? Is it just because he can? Is he exercising some power base and does not really believe in it? It’s neither of these things, and he was so passionate about Enigma. He was also very present, and made sure things were being done properly. It was bloody hilarious, filming in a muddy field with Mick Jagger watching, standing in a pair of wellies."

She also shakes off the furore over a so-called "exclusive" interview by a tabloid newspaper, when her remarks were simply taken from a press conference she had given to 80 members of the Hollywood Foreign Press on November 13. "It was a bit cheeky, really," she says. When she complained, the headline declared: "Kate Winslet disappears up her arse". "If I think about it and let it chew me up, then they have won," she observes, with a shrug. "My family and close friends recognise the inaccuracies and know the truth. When you strip away all the crap, that is the only thing that matters."

She spent Christmas and new year with her parents - and four generations of family - at their home in Reading, Berkshire. "It was great to be around those who know you best," she says. "We have all grown up now. Even my little brother has just turned 21. My sister, Beth, has a little boy, Georgie, who is 13 months. And my grandmother, my mum’s mum, a fabulously with-it lady of 93, was also there. You don’t have to make any apologies for anything in their company. They know and love you for who and what you are."

What Kate Winslet remains is a terrific actress and down-to-earth woman who has been more open and upfront about her own life than many of her contemporaries.

Winslet is now back in the London flat that was her home before the fame and Oscar nominations for Sense and Sensibility and Titanic. Houses in Cornwall and the home counties have been sold in the wake of the divorce. "I remember that, after Titanic, people were telling me: 'Your life is going to change,'" she recalls. "I was very defensive, saying: 'It is not. Why should it?' I am no longer defensive, but I lead my life in pretty much the same way as I have always done."

As for the future, there are no firm plans. "I am reading scripts and waiting for something that sets that fire burning," she says. On the possibility of another marriage and more children, she is more circumspect. "At this point, I would not say yes, and I would not say no."

It’s just a personal feeling, but I’ve always liked Kate Winslet, and I don’t think you can keep a good gal down. Remember when David Beckham went from hero to villain after being sent off for kicking an opposition Argentinian player in the World Cup? He ignored the backlash, just returned to doing what he did best, and now he’s a hero again. "Yeah," says Winslet, with a smile. "And I didn’t even kick anyone."