I often wake up confused about what day it is, and a kind of resurfacing process kicks in. Who am I? Where am I? I struggle to get up at 7, and ideally I’d like it to be 8. Pre-children, I woke with the shape of the day in my head. Now I have to immediately consult my diary. I’d be lost without my lists — I really could forget to pick up a child from school.

There’s always a terribly guilty tussle over whether to get up quickly and wash my hair and take the kids to school, or have a nice lazy breakfast in my pyjamas, then wave them goodbye with their nanny. I probably take them in twice a week, which I’m ashamed of, actually. I have three children at boarding school and two at home, but it depends on what time of year it is. I could have two children at home and be working a normal day, or I could have five children at home and be out in Japan. I’ve had to make the house work whether I’m there or not, and really good help is my extravagant present to them and myself. I have a nanny, Lisa, who comes at 8am, and a housekeeper, Mia, who does all the cleaning, washing and ironing. She’s like everyone’s mother — the children started to call her Mummia, which I sort of don’t mind, because she’s my Mummia too. There has to be someone else there they can rely on.

My shower is an important time for sorting out ideas that have been flying round my head. I have very little time to shop, so I’m quite ruthless about having outfits which I wear in rotation. If I feel nice, I’m a nicer person. My absolute luxury is not to rush, and as I work all hours I don’t feel bad about it. I don’t eat breakfast, because if I do I’m ravenous all day, but I have to stop for a coffee at Daylesford, the organic shop near us, which drives James mad. He’s always champing to get to the office.

Crossing the river to Battersea, I mostly think how lucky I am to be living in London, to be doing something I love. And I still feel intense joy at owning a new bag. I might use five or six from each collection, and I do like to swap them around. I’m a bit of a filing-cabinet lady; I have a system of colour-coded zipped pouches stamped “receipts”, “currency”, “camera”, but there are always a couple of Action Man legs in there too. I think it’s a very girlie thing to segregate and organise and hoard.

I had no idea that I’m Not a Plastic Bag would take off like it did. The launch in the States was beyond belief. Thousands of people camping out. Maybe it just captured the spirit of the moment. I send all my design team out on inspiration trips. I’m influenced by things as diverse as a colour or a street lamp or the way someone walks. You’re out and about and suddenly you realise you’re obsessed with the emblem on a museum door handle. The team see all the shows and gather vintage pieces like fabric and buttons. And we boil up ideas and come up with sketches. It all sounds very fluid, but actually it’s booked into the diary in cement. We choose colours and materials, order samples and do the pricings — and nine months to the day later the line is ready for the shows in New York, London, Paris and Tokyo.

I have to be quite strict about food. My PA, Kate, will say, “You haven’t had lunch,” and she’ll bring me chicken and broccoli. She looks after me, and I look after her, poor love. Sometimes it really does get too much. In June, it’s the Royal Academy party for 1,400 and I have two children’s parties — themed, of course, because I can’t help myself — all the end-of-term shows, and the final edit on the new collection. It’s brutal.

When I get home at 6.30 the children are in the bath. I put the kids to bed then head for Matt Roberts’ gym. It’s real me time, a mixture of exercise and therapy, and I always feel better for it. I can’t cook. I find it exhausting and it gives me mouth ulcers. James’s first wife died, and I inherited my first three children when I was very young. I cooked spaghetti once, which was revolting, and Tia piped up: “I know you tried, but we can’t eat this.” And I burst into tears, because I thought: “I can’t even get this right.” Now I just admit it’s not part of my skill set. When James is away, day 1 and day 2 is take-out rotisserie chicken, and day 3 is cornflakes.

We’re often out four nights a week. James has amazing stamina, but I do get terribly run down. I slightly fall to bits at weekends. I might have a massage, which keeps me sane; or go for a walk. My big frustration is that I spend too much time eating in fancy restaurants, when I long to go to a life-drawing class or do the Jack the Ripper walk round London. I belong to an erudite book club full of scarily educated women, even though I hardly read. The last one I didn’t read was Restless, by William Boyd.

I don’t have much of a beauty routine, other than some Sisley face cream. The fact is, I’m always desperately keen to get into bed and go to sleep. Some of my best design ideas come to me in the early hours: shoes in gold kid, with ankle straps, appear in the air with absolute clarity, like photographs. But if I don’t get to bed early enough, my day is not coloured as brightly. My husband tuts and says it’s all in the mind, but it’s not. When I haven’t had enough sleep, everything is a problem.

Interview: Caroline Scott. Portrait: Domenico Pugliese

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