It's just me and my boy

If you turn the dial down to 30 degrees you use 41 per cent less energy, says Ariel. I tell her I didn't know that. " Neither did I!" Christensen says. "I feel the situation in the world is rather hopeless sometimes, and that the individual person might feel, 'There's nothing I can do anyway, just me alone', but, actually, there is."

Christensen still feels like she hasn't achieved much. "I'm very proud of a lot of things I've done, but I'm not completely over the moon. I work with this one organisation called the Chernobyl Children's Project International, and the woman who founded it and her husband have given up so much of their lives.

"For the past 20 years they've been pulling out kids from the contaminated area and organising operations to save their lives, or helping them die, if that's what's required, and unfortunately it still happens all the time. They had to sacrifice having kids themselves because they have been so exposed to the radiation over many years.

"When I hear about that I think, 'What the hell have I achieved, if these people are doing that?' It's kind of incredible, you can model clothes for 15 years and people will recognise your face and because of that they might listen to something like this."

What most people remember about Christensen is that she went out with the late rock star Michael Hutchence before Paula Yates swiped him. But I have had to sign an undertaking not to ask questions about either him, or being size zero.

"Oh, the only reason it's not fun to talk about it is that in every single interview it's the thing they write about," she says. "I can talk about him for days, because, of course, anyone you've had in your life who's an amazing person and has had a huge effect on you, you can talk about. But I just find it so dull when journalists write about the same subjects over here - the relationships, the money thing, the fame thing."

She went out with Norman Reedus, American-born father of her son, after Hutchence. He acted in four films, then modelled in a Prada campaign, then went back to acting. They were engaged at one point, but never married.

"I'm very realistic. You go through relationships that usually break up," Christensen points out. "How the hell can you promise yourself to someone for the rest of your days? It's a beautiful thing, a romantic thought, but I also cry at weddings because I think, 'Oh God, those poor people!'"
She catches herself, perhaps thinking she is sounding negative. "I just think to go through a divorce would be so nasty that I wouldn't want to chance it, because I'm probably a bit pessimistic about certain things - though with a humorous sort of twist. And also, because we've lived such an independent lifestyle it's hard to let someone in and open up." She means the supermodels. Why is it hard?

"Well, I've never lived with anyone," she says. "So to physically let someone into your space, your home, can be quite difficult, because when you travel as much as I do and do the strange job I've done for all these years, there's no routine to your life."

But it might be nice to live with someone, to come home and there's a light on? "Absolutely. But I leave one little light on..." - she breaks off, now realising she is sounding almost tragic.
"The sad reality of it! No, I really feel good being alone. I really treasure being with someone and all the cosy things you do in couples. But just as much as I love those things, I know there's going to be a point where I feel a bit claustrophobic, that I have to wake up with this person every day."

This opens a whole new scenario to me - that being a supermodel can, ironically, trash your love life. But Christensen does have a dark side. "I think everyone has dark moments," she agrees. "Or lots of people I know. Two or three days here or there where you feel it in your body, creeping up your spine." Does having a child help?

"Yes and no. Because having a child exaggerates everything, so if you're having your down periods you think about your love for your child and all the horrible things that can happen to a child. And the love between me and my son makes me ecstatically happy, but it also at times makes me melancholy, because it's such a beautiful love, almost like a pain."

Meanwhile, the Christmas after next - she was born on Christmas Day - she turns 40. That's a big one if you're a supermodel: the late Gianni Versace said Christensen had the most beautiful body in the world. "Of course, when your body has been the subject of so much interest because of my job, I could freak myself out over how we physically change," she admits. "But I'm more comfortable and happy with myself than I ever have been."

Before I go, I ask if she has a nanny for Mingus. "No nanny. I deal with it the best I can," she says in what later strikes me as a very characteristic answer.

"He is with me or his father, or if we both have to work my parents fly in. It's amazing how he's a full-on little person now, with his own opinions, his own ideas. They're so smart at this age - because they haven't been completely manipulated by the outside world - which happens to us all."