Anya Hindmarch: Dave's got a brand new bag lady

When Anya Hindmarch says that she loves Baroness Thatcher, I want to run. "She has been such an inspiration," says the designer, and I want to leave her sitting there in her boardroom, calm among scented candles and £600 handbags, and flee – down the stairs, past the willowy beauties in the office and out on to the streets of Battersea. I want to find a betting shop, and say to the man behind the counter, breathlessly: "A fiver. On Dave. To say he loves Maggie."

If he can make sense of that, the odds will be high. David Cameron, Conservative leader, often seems embarrassed to be in the same party as the elderly, wild-eyed Baroness Thatcher. But here is Anya Hindmarch, the nation's most fashionable designer of handbags (go on, name another) who is a good friend of Dave and the glamorous organiser of this Wednesday's Black and White Ball – the glitziest night of the Tory year, when the New Cameronians will strut their stuff – enthusing about her debt to the Iron Lady. "I started my business young," she says. "I was 18. Lady Thatcher was pushing the nation to say, 'Get on with it. Get out there. Get going.' Everyone was buying shares ..."

She makes it sound as thrilling as naked skydiving. Perhaps it was. Plenty of people came down with an equally nasty bump, but that's forgotten now in Tory circles. So was Thatcher, but watch this space. As she speaks I think, hmm, Dave must be for turning.

Anya Hindmarch knows what people want before they do, you see. That's her job. The woman sitting straight-backed at the big white table, dressed in black, is also an alchemist. She sniffs out moods, plucks fancies from the air, and combines them with leather and cloth to create objects that sell for a lot of gold. If she says Thatcher is cool again, she's cool again.

Twice Glamour magazine's Designer of the Year, Hindmarch will have 60 shops around the world by the end of this, her 40th, year. Madonna carries her bags, as does Scarlett Johansson. To those of us who can't tell a Prada from a Lada, she is best known for I'm Not A Plastic Bag, the £5, limited-edition tote that caused near-riots when it went on sale last year. The unbleached cotton bag was meant to help save the planet by easing pressure on landfill sites, but also made her a household name. On the day of release, 80,000 people queued to buy one.

"It was unbelievable," says Hindmarch, but that's false modesty. "We planned to get as much drama and hysteria into it as possible." Why? "I wanted to make people aware that doing what I used to do – going to the supermarket, taking 30 plastic bags because I've got five children, going home and putting all the bags in the bin and, ultimately, the landfill – is stupid."

The queues were replicated in the States and around the world. In Taiwan, 30 people were hospitalised in the scramble. "It was awful." Sales of that bag have now stopped, except on auction sites. But how do you plan for "drama and hysteria"?

"The fashion formula," she says, quietly but quickly. What's that? "If a designer who would normally sell a bag for £500 suddenly sells one for £5, the access point to the brand is much lower. Add an incredibly important cause that everyone is starting to think about. Get Keira Knightley wearing it, and other beautiful chicks who are very influential. Then there is scarcity value, which was not manufactured – we just had no idea how many we would need. That's quite a potent formula."

The attacks on her were potent too. The bag was made in sweatshops, wasn't it? "Categorically not. It was made by a fantastic factory which makes for the biggest American companies and is audited externally every month." Shipping it here from China was hardly planet friendly though? "That's naïve. We're not going to be able to make something that will create the awareness we need for £5 in England. My God, I wish ... but we can't." But some shops sold it in plastic bags, didn't they? "No. We told the stores not to let anyone do that. Some customers did take a plastic bag from the counter because they had been queueing all night for their girlfriend and didn't want to get a mark on it. You want to smack them, but there's nothing you can do about it."

The bottom line was the message. "People who bought one are spreading the word. We did something I am proud of." The transportation was carbon offset. "I'm not convinced by all that, but there are big wins we can make. Let's focus on those."

That sounds very much like Dave. "Oh God. Stop it!" Like Cameron, she grew up in a happy family, has lots of money and lives in an exclusive area (Belgravia, in her case). Like him, she has the ability to make you forget all that when you meet her. You can almost see her lugging shopping from Sainsbury's into the back of her Land Rover, without a nanny or personal assistant in sight. Almost.

"I'm thinking Jude Law," she says on the phone to one of her sons, who wants to know how to get his hair cut. Hindmarch is married to James Seymour, a director of her company. He was a widower when they met and had three children already. They have had two more together. "Actually," she says, flicking the phone off, "I'm thinking of Jude Law pretty much all the time!"

She's nice. That's disconcerting. The girls downstairs stared as if they'd never seen a slightly overweight, poorly-dressed man before, but she even apologises for eating a quiche and a mountain of broccoli while we talk. She's deadly serious about business, but admits with a laugh that fashion is "a bit frilly". It's a form of brainwashing isn't it? "Yes," she says cheerfully. "I'm fascinated by it. We will look back on this era, with those stick-thin women with those huge lollipop heads, and go, 'What was anyone thinking? How could that ever be attractive?'"

That's Anya Hindmarch talking. The name on the bags the stick girls carry. The daughter of a self-made man. Born in one of the posher parts of Essex, she didn't go to university after convent school. She went to Italy to check out leather goods, and came back with a handbag that "all the cool girls were wearing". She sold them – thanks to a friend of a friend – through Harpers and Queen. Hindmarch still designs the understated, handmade bags herself, from whatever inspires her. "Fifties architecture, at the moment."

But come on, why bags? What is there to get hysterical about? "I love bags," she says, "because you don't have to try them on, you don't have to be a certain size, they can completely alter your mood." Really? "Ha! It's weird talking to a guy about this. A girl spends a ridiculous amount of money on a handbag. It's madness. But why? They are mood altering. When an actor gets into his shoes, he gets into the part. It's the same thing for a girl with a handbag," she says. "It is a form of self-expression, which is very important in life. It's showing our colours. It's tribal."

How does she square an environmentalist's dislike of waste with blowing a fortune on a bag? "Look, there's a huge industry behind this. There's an awful lot of people downstairs I employ, all of whose families are supported. That is the way the world works. I'm a commercial girl." And a modern girl. A leading member of the "new gang" of successful, young and fashionable Londoners she says are committing to the party. The environmentalist Zac Goldsmith was alongside her on the organising committee for the Black and White Ball, but many more creative and successful friends have "come out of the closet" in recent months, she says. "They all agree we need a change."

Guests will pay at least £450 (and up to £35,000) for a ticket. Charles Moore, the former Telegraph editor, called last year's party "an ordeal" featuring "someone dressed like a Russian prostitute sprawled on a bar playing an aluminium guitar". But that wasn't organised by Hindmarch. "I hope it will be elegant and beautiful. We've built eight Chelsea Flower Show gardens inside a tent."

This being New Tory, the flowers will be exquisitely tasteful, but not just for show. "We're donating most of the plants to schools and parks through the Conservative Social Action Programme. That's probably as much as I should say on politics." Probably. Let's press on anyway. It's the Notting Hill Party now, isn't it? "The people I have spoken to are not the Notting Hill gang," she says, a little crossly. "They're from all sorts of backgrounds. People who have got off their bums and done something."

Very Thatcher. Hanging on the wall of her office in a frame is a handwritten letter from the baroness, thanking Hindmarch for the navy blue handbag she sent as a gift. And guess what? Dave loves Maggie now too, it turns out after this interview. To many people's surprise, on Thursday he presented this "towering figure" with a Great Briton award. Anya Hindmarch knew what was in the air. It's what she does. Shame about the bet, though – with a big win, I could almost have bought a handle.