Penny's already planning baby No2

The "kids" range from 11-year-old Liam to 25-year-old Kimberly ( Stewart also has a 41-year-old daughter from a teenage relationship, but has no contact). Lancaster, 34, talks as if they're one big happy family, but it wasn't always like this. When she first arrived on the scene, it was difficult, particularly with Stewart's older daughters (Kimberly is just nine years younger than Lancaster).

"You have to be patient and do a lot of listening and be yourself," says Lancaster. "That was at the beginning, but for a long time now it's been wonderful. I've got a great relationship with them all. We talk about school, boyfriends, everything."

The former wives haven't been so easy to bond with, however. Rachel Hunter, Stewart's last wife (he also married Alana Hamilton and had a daughter with Kelly Emberg), famously walked out on Rod to "find herself" six years ago. Since he started seeing Lancaster, Hunter has always been in the background, sometimes with a catty comment or two. Hunter once apparently called Lancaster a man (it's true that, at 6ft 1in, Lancaster is giant-like, but I've met Hunter before and she's less feminine), then signed up to be the body of a lingerie company which had previously used Lancaster in its campaigns. Hunter has hinted in interviews that she wanted Rod back.

"I don't know about that," says Lancaster. "Everyone is entitled to their own opinion."

Has Hunter congratulated Lancaster on the pregnancy? She squirms. "I haven't spoken to her recently; we talk a lot about the kids. Those are the only conversations we have really."

Perhaps until now, Lancaster was always seen as Rachel mark two - she dyed her hair blonder and wore shorter skirts when she met Stewart. I ask her if she thinks having a baby and getting married has cemented her place in Stewart's life and family. "Definitely," she says, "the kids like the idea that there's a security. We've been together for a long time, I haven't just turned up on the scene. The fact that Rod proposed and we're having a baby, it's brought us all together. But I never worry what other people think."

Lancaster met Stewart at the Dorchester hotel when she was still a student. She had gone out with some girlfriends and they dared her to go over to Stewart to ask for his autograph. She did and she told him she was a photography student. He invited him to one of his concerts to take photographs and took her number. His relationship with Hunter was breaking down and he called Lancaster nine months later. Her own long-term relationship - she had been with a banker for 10 years - had come to an end.

Suddenly she was going out with Rod Stewart. "I didn't really have an opinion of him, other than knowing he was a rock star who wore leopardskin trousers and sang Do Ya Think I'm Sexy? But having met him, I realised what a gentleman he was, really charming." Is it hard to believe they're now getting married and having a baby? "I know, I know!" she says, laughing (on the way home I think about this; even if I was blonde and leggy, it would still be absurd to imagine I could ask, say, George Clooney for his autograph and end up marrying him).

Does she mind the age gap? "I think that's a benefit in our relationship. I've always been attracted to older men. They tend to have more oldfashioned values than younger generations. Rod is a true gentleman. He's got a great sense of humour, just like my dad. If you have a great relationship with your father, you want someone who will match up."

But when she's fortysomething, he'll be in his seventies. "He'll still be gorgeous. He'll be running up and down the football pitch and still be singing. He'll still be Rod."

Lancaster grew up in Chigwell. Her father was a lawyer, her mother did secretarial work. At school, she was bullied mercilessly because of her height. "I had a strong upbringing and a close-knit family. I never felt alone. When I was bullied at school and I had insecurities because of that, my parents told me not to feel angry at people, to feel sorry for them and to be proud of who I am. They reinforced that at an early age.

"I'm not as confident as Rod wishes I was and he has often said I should be tougher, more resilient against certain things, and I think, 'I'm just being me.' Throughout my life, that's worked - if you smile and try to be a good person. I can't get angry. I can't think of anyone I would want to say a bad word against. It's bad karma and disturbs your inner energy. It's much healthier to be happy and positive."

She continued with her photography and modelling career and was recently commissioned by Ariel, the washing-powder manufacturer.

"My priority has always been my relationship," she says. "I'm not really a career-oriented person. I love photography and modelling and my fitness [she is a trained instructor and has made fitness videos], so I will keep those areas going, but now I'm going to be a mummy and that's the best job in the world. Looking after a baby and a family, being with Rod, that's what I want. I want to carry on going on tour, because I know that's what he loves. Eventually, I'll get back to my work."

Her mother is going back to their house in LA after Christmas to help with the new baby, then Lancaster will start thinking about their wedding and a second baby at some point, "but we'll have to see how the first one goes. This is the moment I've been waiting for, for a long time. I've got everything I've always wanted."

She may not be the poster girl for independently minded career women but there is a genuine sweetness in Lancaster that is rare indeed. I always assumed it was Lancaster who got lucky with Stewart (if ageing millionaires happen to be your thing). Now I wonder if it isn't the other way around.