When Dorothy Byrne described Boris Johnson as “a known liar” and compared his media strategy to Vladimir Putin’s, she must have known she would get into a dispute with Downing Street. “Oh, I’m never in a dispute with anybody,” she answers with a speed that seems almost combative. “People can have a dispute with me. But I don’t have disputes with other people.” This sounds like the beginning of a dispute in itself, but perhaps she thinks I am having one with her.

Byrne, the head of news and current affairs at Channel 4, made headlines herself last month when the prime minister’s office cancelled an interview with one of her teams following her outspoken comments during Edinburgh television festival’s annual MacTaggart lecture. According to the editor of Channel 4 News, Ben de Pear, a team of journalists was invited to Biarritz to interview Johnson during the G7 summit – only to find access denied.

I bet Byrne, who once described herself as “Outraged of Horseferry Road”, was furious.

“I’m not outraged by politicians. No. On this issue [lack of access to politicians], I would say I am genuinely concerned about the state of our democracy.”

Anyhow, the interview was not necessarily cancelled. “With these things you never know,” she says. “Are they going to do the interview, are they not? Sometimes they do it and sometimes they don’t. So, do politicians cancel interviews or were they never quite definite anyway? That’s what I would say.”

This seems like a major U-turn. After all, Downing Street briefed on the reasons for the cancellation. “This is all part of the to. And fro,” she says. She has a habit of interspersing speech with heavy pauses, so her sentences seem to respawn themselves halfway through. It is a bit like talking to a politician. She didn’t take the cancellation or non-cancellation personally?

“Oh, I never take anything personally!” she says.

She thought hard about calling Johnson a liar. “It’s not a word I bandy around. But every now and then you just have to say something that’s true. I’ve said it. I don’t need to repeat it. Nobody has said that what I’ve said isn’t true,” she says. She refutes any suggestion of partiality. “If we don’t all agree that truth has a primacy in democratic debate, where do we end up?”

Besides, “one particular interview in Biarritz isn’t what it’s all about. It’s about something much bigger.”

Byrne grew up on the outskirts of Blackpool, having moved from Paisley at the age of six. In summer holidays, she worked on the trams; each mill hired one, and it would only pick up people from that mill. “Imagine what a cohesive group those people were,” she says.

But no one knows where they belong any more, she says. And just when an explanation is most needed, politicians are largely unavailable.

A male colleague reached up my skirt. I just took his hand and handed it back to him

Byrne’s father, who worked in war pensions, always voted Conservative (as did her mother), “because he said: ‘The Conservative party is the party for the middle classes and I’m middle class.’ But now people don’t know who to vote for.”

Taxi drivers, when they discover Byrne’s occupation, ask her whom to vote for. They want to know what is going on and she sometimes doesn’t know and has to ring up Gary Gibbon, the political editor of Channel 4 News, from the cab to ask. “Not everyone is lucky enough to have a Gary Gibbon they can ring up.”

Byrne’s was not a political household, but at her convent school in Blackpool the nuns wanted her to be an MP. She won the regional debating competition and the nuns thought she could win the national, “which was, and still is, almost always won by St Paul’s or Eton or City of London. So they brought a man from the Christian Brothers school down the road to teach me to be confident. This probably has affected me,” she says ominously.

She had to go alone into a room with him and stand on the table. “A great big Victorian table,” she says. “He would give me a subject to talk about. I didn’t get any warning. And as I began talking, he would go: ‘Fuck off, you stupid little bitch! Fuck off!’ and I’d go: ‘What?’

“He’d say: ‘It doesn’t matter what I say. Keep talking!” She was 16. “And, actually, during my career, men have said to me: ‘Why don’t you fuck off, you stupid little bitch?’ and I say: ‘I’ve been trained. You can’t affect me.’ I just keep talking.” She pauses. “And we did beat them.”

This explains why Byrne is so hard to interrupt. “Maybe I’m defensive, though. That could be why,” she replies. She folds her arms, which certainly makes her look defensive. “All right, no, yeah,” she says, unfolding them and ostentatiously pinning them to her sides.

In a sense, Byrne’s working life has always featured adversaries. On her first day at Granada in 1982 , she was told by her female boss that a male colleague would sexually assault her. Two days later, he reached up her skirt. “I just took his hand and handed it back to him,” she says. There was no point in reporting it. She never considered leaving.

‘I felt I mustn’t be seen to be an older woman with a medical problem that was affecting my work.’ Photograph: Linda Nylind/The Guardian

“That’s what life was like in all workplaces in Britain then. There would have been no point leaving Granada and going to work somewhere else because that sort of low-level sexual assault was absolutely common.”

Later, when she became a mother – she was 45 when she had her daughter Hettie, a donor child – she had a job as a freelance editor of ITV’s The Big Story. There was no maternity pay, so she hired a live-in nanny and soon returned to the office.

“What interests me now, with men that I work with – and I’m not saying that they shouldn’t do this,” she adds. “Men will say: ‘Oh, my child’s in nursery and they’re not well, so I’m going home now.’ And they just go. In many ways, I think that’s great,” she says, with a hesitation that suggests there may be ways in which she thinks it is not so great.

“I would never have felt that I could do that. And I look back on the fear that I felt – if I had said I had to go home because of my child, that would make people think less well of me professionally.”

Is she glad that parents have more leeway in her workplace now? “Definitely. And I think it’s great that it’s men who say: ‘I have to go,’” she says. But she still sounds regretful. I wonder if this might be because, looking back, she feels sorry for her younger self. “Yes I do. I think: why? But it was like that back then.

“I was asked to write an article about having a donor child for a national newspaper,” she says, leaping to another moment of painful, enforced silence. “So I wrote this article. Channel 4 said I couldn’t have my name on it because it would undermine my reputation.” They thought it was shaming? “It was very unusual at the time. I wasn’t ashamed. I was proud. I was a forerunner when it was not so common, so I thought it was terrific. But why did I go along with it?” she says, sounding exasperated. “I look so strong and powerful and I was the girl who stood on that table and I beat all those public schoolboys and yet I went along with all those things.”

Even 10 years ago, when she was diagnosed with polymyalgia rheumatica (PMR), an autoimmune disease, and giant cell arteritis (GCA), she felt the need to hide her illness – even though she was already Channel 4’s head of news. When the steroids exhausted her, “I would walk away from my desk and go to the car park and sleep in my car for 45 minutes. And then I’d go back to my desk.

“I look back on that now and I think, why didn’t I say: ‘I’m really tired and I’m going home to have a sleep’?”

Well, why didn’t she?

“Yes, why didn’t I?” she says. “You see, I related to the fact that I had had a live-in nanny and didn’t take time off. It’s that feeling … I mustn’t be seen to be an older woman with a medical problem that is affecting my work.”

Is she cross with herself? “No, because I think you shouldn’t be cross with yourself, because that’s negative,” she says.

She has no qualms about disclosing her age now. “Sixty-seven.” She says it without a beat. But for a long time she hid it. She was in her early 50s when someone at Channel 4 confided in her that they thought a colleague was too old for her job. The colleague was 47. “So I thought: ‘I’ll just never say anything that reveals how old I am.’”

Then, one day, the Evening Standard got in touch, hoping to include Byrne in its list of the 1,000 most influential Londoners. But she would have to specify her age. By then she was 56. The journalist Peter Oborne happened to be making a film for her. “So I said: ‘Peter, I’m thinking of coming out as my own age. What do you think?’ And he went: ‘Hmm. I like it, Dorothy. Une femme sérieuse.’”

When the article appeared, “I said to Kevin Lygo, who was my boss at the time: ‘Oh Kevin, let’s see if you are one of the thousand most influential people in London …’” Byrne mimes flicking a newspaper. “‘Oh look, you’re not!’ And then I said to Julian Bellamy [another colleague]: ‘Let’s see if you’re one of the thousand most influential people in London ... Oh look, you’re not! I tell you what, just for a joke, let’s see if I’m one of the thousand most influential people in London. Oh look, I am!’”

It is a beautifully scripted scene. As they were saying: “Huh, how did that happen?” Byrne pointed to her name in the paper. “So they would see my age, but they would see it at the same moment as they were feeling irritated that they were not influential.”

The really difficult period was in her 50s, she says, when many women dye their hair and wear lots of makeup. “But then at a certain age you just think: ‘Well, I’m so old, it’s OK. I can be old.’”

This doesn’t mean, however, that she plans to retire (“You wouldn’t ask a younger woman that, so don’t ask an older woman!”). As she says, the working world needs older people. “You see things in a different way.”