Even now, Libby Purves buys her own theatre tickets

Libby Purves doesn't bother with pleasantries, like saying "goodbye" at the end of phone calls, or even after we emerge from an hour-long interview when she simply turns and lollops off.

But in other ways, the 60-year-old host of Midweek, the mid-morning, middle-brow, middle-class Radio 4 show, is the embodiment of pleasantness. Mumsy and round-faced, she is free of vanity, and hates it in others. She makes a point of never calling the programme "hers", even after 27 years.

The mystery is how this matronly figure has earned one of the smartest CVs in the media, working as a columnist, presenter and editor. She was once described as "at the top of the second division, always refusing promotion", but six months ago she agreed to replace Benedict Nightingale when he retired as theatre critic of The Times, where she is already a columnist.

It prompted some sniffing from people who asked if she went to the theatre. "What a ridiculous question!" she snorts. "I used to go to the theatre twice a week off my own bat. My family are all theatre-mad. It's in the DNA." As if to prove her point, Purves later shambles across the road to see a matinee of All My Sons, "just for fun. I want you to know I bought my own tickets".

Nobody could doubt her probity: Purves is as proper as a church fête, though she peppers her conversation with words like "crap" and "bollocks". She was "a really crap actress" at Oxford, though the experience fuelled her love of theatre. She got a First in English, so should, she laughs, be all right picking up literary nuances. "I would be upset it they gave the job to someone who had never been to the theatre and just wanted to write glib pieces. But I've got the right background."

Since the most demanding part of being a theatre critic is writing to tight deadlines, 40 years as a journalist will help. She joined the BBC as a studio manager in 1971, becoming a reporter and then presenter of the Today programme – at 28, the youngest, and the first woman. But the confrontational style was not for her: she is much happier with Midweek's round-table chat.

She has written over a dozen books, been a columnist for several papers, and has been awarded the OBE for services to journalism. Her telling of that experience is typically self-effacing – persuaded into it by her family, who wanted to meet the Queen, she ditched her customary trousers for a dress, which "turned out to match the swirling pink carpets at the Palace".

For someone with no interest in fashion and a frank disdain for smartness, she has gone down in Fleet Street lore for having the ultimate William Boot moment when, in 1983, Condé Nast begged her to edit Tatler. The experience was "a merry hell", and she left after six months.

But the greatest mystery is how she has survived so long at the BBC, notorious for dropping older female broadcasters. "It happens in television, which I have never liked," she says. "Radio is different." It may help that she has always been freelance, and her contract with the BBC is renewed annually. "One day they will just not renew it. The same is true of The Times. But journalism has always been like that."

Purves is the eternal pragmatist, always making the best of whatever comes her way. This buoyancy faced its hardest test four years ago when her 23-year-old son, who suffered from chronic depression, hanged himself at the family home in Suffolk. She is reluctant to discuss it, though she says she and her husband, Paul Heiney, now spend much of their time in a tiny house in Oxford, a city beloved of their son: "It's more cosy to be there."

Later, as we move to leave, she suddenly can't help talking of him. "When I got the theatre job I thought, Nicholas would really approve of my doing this. As long as I did it honestly, as long as I was honestly open to the plays, even if they weren't necessarily my kind of thing. He'd like that, he'd approve, he'd nod, he'd be glad, and he'd have come with me a lot, and his advice would have been rather better than mine, because he was very perceptive. So yes, you do go on, because the alternatives are – defeatist."

A month into her new job and Purves is still very much the new girl. After her first six reviews she was formally accepted into the tight-knit circle of critics, who, despite a reputation for bitchiness, are "not nearly as bad as people say". My last question is to ask what she makes of Matthew Norman, the Independent journalist who says there is no more depressing 12-word sentence than, "And now on Radio 4, it's time for Midweek with Libby Purves".

"I have no idea why he says those things," she says with a sad shrug, "I've never even met him. I suppose some journalists hope that if they write nasty things people will pay them attention."

And with that warning ringing in my ears, she's off.