You don’t so much interview Dame Vivienne Westwood as end up as an audience to her monologues. Britain’s most famous designer is promoting How to Change the World, a documentary about the birth of Greenpeace, although she doesn’t actually have anything to do with the film itself – she just wants to draw attention to climate change. She is fanatical about drawing people’s attention to climate change. She has a blog, Climate Revolution (formerly Active Resistance), and works with Greenpeace regularly. She has spent the summer appearing at festivals, talking about climate change. The day after we meet, to much fanfare, she rides a white tank up to David Cameron’s constituency home in Chadlington, Oxfordshire, to protest against fracking. The pictures have been endlessly shared on social media. It was a brilliantly arresting stunt.

Westwood is one of the founders of punk, who famously didn’t bother to wear knickers when she collected her OBE from the Queen, so it’s no real surprise that she has driven a tank up to the prime minister’s house to tell him off. There’s a campaign running on her blog at the moment called Politicians R Criminals, which intends to remind everyone that “you’ve got to be scared of these people”. She doesn’t much like the establishment, with the exception of Caroline Lucas, and the Green party. When I tell people I’m going to meet her, those who have encountered her before describe her as formidable and combative. She will talk and talk, they say.

Westwood’s PA, Cynthia, seems exasperated before we even begin. “She’s a loose cannon,” she says with a shrug, while we wait for her to arrive, in an office in her Battersea HQ. Westwood finally enters the room: she is 74 now, tinier than I expected. Westwood doesn’t have much time, she tells me, that notorious voice quieter than I had imagined, because she has got to visit Julian. I assume she is talking about a friend, until she mentions the embassy. She means Julian Assange.

Westwood outside the Ecuadorian embassy in London, where she visits Julian Assange once a month. Photograph: Jonathan Brady/PA Wire

“I go once a month,” she nods. “I’m really looking forward to it because they give you such a nice cup of coffee at the Ecuadorian embassy. It’s really good. I pick his brains. I think he’s brilliant.” They’ve been friends for years, since Assange invited her to his birthday party. She calls him a hero. I think we might have a debate. But is he a hero? “Well, of course he is,” she says, as if I’ve just asked her if she has any interest in clothes. “He’s got a library of truth that he’s building and checking and it’s a complete challenge to the official view, all of the time. It means newspapers are going to have to be more truthful. It’s really, really good.”

It very quickly becomes apparent that it is impossible to discuss a subject with Westwood. She has her opinion, and that’s that. That’s not to say she is unfriendly; she really does just talk and talk. She continues her point about Assange being a hero for another five minutes or so. Before the interview, Cynthia gave me some advice: speak up, and speak at her, because she’s slightly deaf. Try to keep her on track. And don’t interrupt.

I would like to keep her on track, so against Cynthia’s other warning, I try to interrupt, but it doesn’t work – she steams ahead, regardless. She spirals from how newspapers lie, to the migrant crisis, to climate refugees, to how the entire financial system is “a war economy”. She drifts in and out of threads of thought. There are people milling around the room, about to leave, and they’re distracting her.

“Just understand that it’s a problem for me,” she says, returning to how the media is propaganda. “Anyway, you’ve got the point, haven’t you?” she asks at the end of it, though I am not sure I have. “I’m just hoping that people will understand that climate change is on our doorstep, it’s here, and that we’ll all be migrants soon. It’s something we might still have the chance to stop.”

Does she trust any politicians? She says Lucas, of course, because Lucas is a big supporter of proportional representation, for one thing, and Westwood thinks PR is a necessary change to the current system. I ask if she supports Jeremy Corbyn. “Yeah, we do. We think it’s a real mistake for him to talk about having clean coal.” She tells a long anecdote about being on a panel with Kenneth Clarke and Liz Kendall, who believe you can have safe fracking, then goes on a detour to talk about arms sales to Indonesia. She comes back to “clean” coal, saying it is an old-fashioned, old Labour idea. “I want to support Jeremy Corbyn but he’s wrong there. I’d rather find somebody better to support, but I don’t know one better, except for Caroline.”

Vivienne Westwood drives tank to David Cameron’s house in anti-fracking protest Guardian

Westwood thinks people vote Conservative because the status quo makes them feel safe. “I just think of my mother. She was very poor, yet her family voted Conservative. And when I first voted, I voted Conservative because I was brought up like that. Of course then, afterwards, I wasn’t voting Conservative. [In 2007, she announced that she would vote Tory; she supported the Greens in 2015.] Until she was 93, my mother always went and voted Conservative. When you asked her why, she said they were good for the country.”

Westwood is notorious for kicking against the status quo. It’s ironic that she’s here to talk about the Greenpeace documentary, because the Greenpeace founders were self-proclaimed hippies, the hippiest of hippies. Flares, long hair, beards, magic mushrooms. At one point in the documentary, the group’s founders roll around in the moss, having a collective epiphany about the interconnectedness of the universe. Westwood is punk incarnate, dressing the Sex Pistols in bondage gear, sticking safety pins through the nose of the establishment. “It was only Malcolm [McLaren, her ex-partner] who was against hippies,” she says now. “That was his idea.” But wasn’t “Never trust a hippy” one of the era’s big slogans? “I actually never said it. I think the hippies absolutely politicised my generation. It was wonderful to find out what was going on, about dictators and what they were up to. I needed to. That’s why I was attracted to Malcolm in the first place – because he understood a lot more about what was going on, and I needed to know what was going on in the world.”

Had their paths crossed in the 70s, might she have been friends with Greenpeace co-founder Bob Hunter and his fellow activists? “I’ve never taken drugs,” she says, as her reply. “I’m not interested in drugs. I mean, I do love alcohol. But I don’t like to think I’m not in control. I’ve had a couple of puffs of stuff, me and Chrissie Hynde, smoking something. That was a waste of time. It just gave me a sore throat. Next time, I started to hallucinate and I thought, oh no, thank you. I don’t want that.” She hasn’t touched marijuana since.

Westwood can be great company. “I was a single parent most of the time,” she says, wandering off-topic on a question about why she thinks she got ahead in life. “Especially living with Malcolm McLaren, because he just denied having anything to do with [the children]. He was only here every now and then, causing trouble.” She does a rapid about-turn without seeming to notice. “He was great, actually. He’d give the children very dangerous things to do, and everything. He was very good to them.”

What does she mean, dangerous?

Westwood with her then lover and business partner Malcolm McLaren, in 1981. Photograph: Richard Young/Rex

“Things you wouldn’t be allowed to do today. When Joe was about eight and Ben was 12, he sent them on a bicycle ride to Devon.” From London? “Yeah,” she says, smiling. “It took them four days. They went to youth hostels, and it all worked out and they got there. See, people thought very differently about their children,” she adds, catching the look on my face. “When it was dark, they got lost, and knocked on a lady’s door to ask the way. She brought them in for a cup of tea and called the police. The police phoned me, and I said, no, it’s quite all right. They let them carry on with their journey. As long as I knew, it was all right.”

Does she think young people are as political today as they once were? “I think less. What do you think, Cynthia?” she asks. Cynthia, who is sitting in on the interview, says it’s half and half. “You either know a lot, or nothing,” Westwood agrees, thinking it over. She says she likes Noam Chomsky’s idea, though says it might be US author and climate activist Bill McKibben’s, about how people think they’re the only ones to hold an opinion. “They think, well I don’t agree with this, but I daren’t say it. I daren’t say that …”

What she then says is incendiary enough for Cynthia to shout that that was off the record, and we all laugh. Unfortunately for Cynthia, Westwood ploughs on, and continues with her theory that 9/11 was an inside job.

We talk over each other: at the same time as she adds with a glint in her eye, “Don’t ask me what I think”, I say: “Do you think that?”

“Well, I do. I should never say this. I self-censor,” she continues, doing no such thing. “Everybody self-censors like that. But when you actually ask people, they say it’s a demolition. How can you lie to yourself about that? Just put it that way. That would be kind to me, if you don’t mind. You have to work it out from there.”

How does she know who to trust when there are so many voices?

“You have to trust yourself. Of course you do. And in the case of the twin towers, my eyes tell me it’s a demolition.”

Cynthia is no longer laughing. “Vivienne, you really have to go.”

Westwood is suddenly childlike. “What shall we do, Cynthia? Ask her to drop it?”

I can’t promise to do that, I say. We’re in the middle of an interview. Westwood is too longstanding a veteran to expect that she can erase things she has talked about at length, after the fact. Later, I search online – she has stated her beliefs on 9/11 publicly before, in a filmed interview for Dazed Digital last year.

And she doesn’t stop, even now. It’s as if she can’t help herself. She talks again about how she is certain the twin towers were demolished. “Most people think it’s not possible for people to be that evil, to do that. And I know that people are very evil. So, I don’t know.”

Cynthia is on her feet. I try to get it back on track. I ask Westwood if, in that case, there is any hope for her environmental activism. We’re here to talk about Greenpeace, after all. “No, but we have to carry on. My next fashion show, I’ve called it Mirror the World. What I mean by that is that you have to understand the world you live in, and you should be a little splinter that mirrors the world. Today you have to understand politics, you have to understand what’s going on as well. I’m never going to talk about the twin towers ever again.”

“Well you’ve just done it,” says Cynthia, crossly. “You’re 45 minutes late.”

“What shall we say to Rebecca?” asks Westwood.

“Rebecca’s already made a plan. She’s going to do it. Which is a real pity,” says Cynthia, frostily.

I have not had time even to think about a plan. The whole situation is dizzyingly strange. “I don’t mind that it’s in there at all. Not at all,” Cynthia continues. “I don’t want it to take the focus off Greenpeace. I think if it turns out to be all about that, it will be a real shame. Because we really want to support Greenpeace.”

There’s a lift in the office. Cynthia has called it up and opened the doors. I think she’s waiting to take Westwood away, until I realise she is waiting for me. I pick up my bag.

“But can I just say one thing I think you should add,” says Westwood and, for a second, I think Cynthia might explode. “That is not the issue at the moment, whether that happened or not. One time I thought it was a big issue. Because I thought it would convince people of what harm people are prepared to do. There are more important things: migrants, climate change. The reason it’s not so important is because so many people in the world accept that that’s what happened anyway. People have absorbed it, internalised it, that’s how the world works. You’d be surprised the people you ask. The most conservative people say, oh yes.”

Cynthia is holding the lift, impatient for this to be over. Well, thank you for talking to me, I mumble, suddenly feeling very sad. One last question comes into my head, and I’m not sure why. I ask Westwood if she likes people. She thinks about it. “I must,” she answers, finally. “Because I keep sticking my neck out.”