Did she always trust his judgement? "One hundred per cent. I wish he was still alive now, because I would be asking him, as well as Trinny, what to wear."
What criteria did her father use? "Shape, showing off the figure. And simplicity as well. He thought simplicity was the art of decoration."
He bought her a vintage lace dress owned by Bianca Jagger. And when she was 22, he invested in a dress by Valentino for a special occasion. "It was beautiful. I could no more put my left leg in it than my body now. I was sitting at this very grand dinner, next to these two very important men." She laughs. I get the impression ‘important’ is an understatement. Susannah mixed in royal circles and was once Viscount Linley’s girlfriend.
"But the dress was very badly made," she goes on, "and the straps went and my boobs were laid bare at the dinner table. It was a really embarrassing moment, and this butler came in with safety-pins." She imitates him weaving between tables with a salver held high. "There was no ‘Let’s do it subtly.’ I had this very high-profile man on this side, doing that strap, and another very high-profile man on this side, doing the other. Meanwhile, I was lifting my boobs out of the soup."
So much for expensive labels. "That was Valentino! They should have f**king done a better job than that. We took it back and they were so unapologetic. If I had taken that back to Oasis, they would have given me a new dress."
They espouse high-street favourites such as Zara and Topshop. But their posh voices, their oh-so-confident manner, suggests more exclusivity. Are their lives really relevant to the lives of the women they help? When one woman suggested on camera that they weren’t, Trinny’s sharp retort reduced her to tears. But didn’t she have a point? "What we say is nothing to do with class or background," insists Trinny. "It’s one woman to another woman."
They share the same body features as other women, adds Susannah. "In our emotional lives, too, we share," continues Trinny. "There were so many instances where men didn’t compliment women. We all share it - not getting enough affirmation from your husband or boyfriend, something to make you feel good about yourself."
Susannah looks thoughtful. "You know what, Catherine? That whole class thing has changed so much. I don’t think we will be asked that question in a couple of years’ time. You look at the new people on the pedestals, the pop stars, the Posh and Becks. They come from very humble beginnings and are aspirational, and they have gone out there and done that. Those are the people who are commendable, not someone who through luck of birth has ended up in a f**king castle somewhere. I don’t think people look up to the aristocracy or rich people. So what if you live in a castle."
She speaks with passion. But a rich person is still more likely to say class doesn’t matter than a poor one. Earlier, Susannah herself had talked about broadcasters with BBC voices having more authority. "We are two posh birds," she had acknowledged. "There’s no getting away from that. Middle-class backgrounds, privately educated, we are what we are. Maybe the reason we have been successful is that our backgrounds have given us a little extra authority."
Precisely. Would we listen to Trinny and Susannah if they came from a housing estate in Slough? And yet there is no doubt that the women they dress look better. Watch them for 30 seconds and the duo can be enormously grating. Too smug, too confident, too opinionated, too much. But watch their entire programme, and their warmth and motivation emerges, their desire to make women feel good. Trinny - rather pointedly - says, "It’s also about how you choose to see people, because it’s easier for you to see them like that."
She is not, she insists, really a posh girl. "My grandfather was Scottish, born in the slums of Glasgow. He worked in the shipyards and then did that Posh and Becks thing of working really hard." He became head of British Steel.
She undoubtedly seems the more prickly, more complicated, of the two. Her father’s marriage to her mother was his second, and she was the youngest of his six children - three from each marriage. "When you are the youngest there is the mixture of wanting to be the centre of attention and feeling permanently bullied by your siblings. Your parents think, ‘We should maybe be nice to this one.’ And your siblings are, ‘Punish, punish, punish!’" She laughs, but it becomes clear that she felt she had to prove herself in her early life.
She lived in France and Germany, attending boarding-school between the ages of 12 and 15, then coming to London for the last two years of state school. (Susannah, too, was at boarding-school, and detested it so much that she would never send her own children.) "I never really had an English upbringing, and probably felt a bit of an outsider because of that," admits Trinny.
But it’s when she talks of establishing her own identity that you glimpse vulnerability beneath that clipped exterior. "I had an older sister at boarding-school and she was terribly popular, and I found that difficult. She was the star of the school and I felt really ugly and not as glamorous as her. All the girls in my school... you had this thing where, if you liked someone, you sent them a little note. They all had a crush on her, so they’d be friends with me to get to my sister. So it was a bit of a relief when she left."
Like Susannah, Trinny’s father influenced her sense of style. "My father is actually far more vain than my mother. He always loves to have everything very chic and polished and fastidious. My mother has never worn face-cream in her life. We had to force make-up on her. She has great bone structure and is a very attractive woman, but she has a total lack of vanity that comes from being Scottish with quite a strict upbringing."
While Susannah was working in the design world, Trinny was working in the City and hating it. "I wasn’t doing what I felt I should be doing, but what other people felt I should do. If you live your life like that you are never very happy." She admits that she became an alcoholic, but believes the cause was genetic rather than emotional. "I have that gene in my family. All I know is that I discovered it in time. I chose to take the positive road. I know what it is like to be in a situation where you think you might not be around for long, therefore you really re-evaluate your life." She has been dry for 14 years.
Now the mother of a one-year-old baby daughter, Trinny had difficulty conceiving and underwent fertility treatment. She became pregnant while Susannah was expecting her third child. The two get professional confidence working together, but also have a strong personal relationship. They have been through a lot together. "When we were in LA, both of us thought we were losing our babies and it was really tough," says Trinny. "We were working very hard and we were both bleeding, and if we hadn’t had each other it would have been dreadful."
They may look like they were born with silver spoons in their mouths, but that hasn’t protected them from emotional turmoil. "You know," says Susannah, "that silver spoon is only money. That’s all. It’s not anything else. And money, let me tell you... you look at all those wealthy people and it is the root of all evil. Too much money is really destructive, and silver spoon or not, we’re still human beings. We still mourn the death of a father, or have a hideous time emotionally trying to get pregnant. We are all affected in the same way."
The death of her father was a landmark for Susannah. "It happened very suddenly. One minute he was absolutely fine and the next he wasn’t there. He was on a life-support machine for two weeks." She smiles. "There’s a funny story I haven’t told before. He was wonderful, my father, in so many ways, but he was quite a snob. My sister and I were with him when he died, and the nurse came up to us and said, ‘I’m really sorry to have to say this to you, but we’re going to have to send out a press release.’ My sister and I looked at each other and said, ‘What are you talking about?’ She said, ‘Well, when King Constantine of Greece dies...’ My father would have been thrilled with that moment. It was such a fantastic way to depart."
She looks vulnerable, almost tearful. When it’s time to go, she says how lovely it has been. So interesting. Trinny, I notice, says nothing. But then Susannah described Trinny as controlling, and Trinny described Susannah as manipulative, which means they both try to get their own way, but one is just more upfront about it.
Susannah is trying hard to address my obvious problem diplomatically. She says I should go shopping before my flight. "No more red and black," she mutters, rather sorrowfully. (source: scotland on sunday, including accompanying image)