Kirsty Wark can stand up to some serious cross-questioning on the subject of fashion. Faced with a three-metre rail of the season's finest skirts in our studio, she is formidably well-informed, identifying Marni and Dries Van Noten at 10 paces. Next, her news instincts hone in on the surprise in the designer line-up, a Topshop windowpane check. (She recognises it because she tried to buy it in store, but it was sold out in her size.)

Like a true professional, Wark has come prepared, producing a bottle of Frenzy, this season's Chanel nail polish, from her bag before (the sign of a woman who knows her shops, this) reeling off a list of what size she takes in each designer's clothes. And, crucially – because you get nowhere in fashion without taking risks – she is up for a challenge. Faced with an immense silk-taffeta Christian Dior skirt of the genus that sent Jennifer Lawrence tumbling at the Oscars, Wark barely blinks before putting it on, pronouncing it "very Liaisons Dangereuses", stepping into shoes that are both high and too big for her (a perilous combination) and swishing across the studio.

The hand-painted organza Dior skirt is unlikely to grace the Newsnight set any time soon, but it is an amplified version of a formula that Wark has made her own. "A block colour on top, worn with a patterned skirt, works well for Newsnight," she says. "It's getting that balance between clothes that feel modern and lively enough to be appropriate for the programme, but that are not distracting."

Wark, who at 58 has worked on Newsnight for 20 years, is proof that – contrary to what sometimes seems popular belief – feminism and an interest in fashion can coexist. "Why would it be antithetical to feminism to be interested in style, in design, in line and colour and cut? Why would a desire to feel good about yourself, to look modern, be at odds with feminism? Look at Simone de Beauvoir! She looked fabulous."

Naturally, I'm with her on this one. Surely feminism should allow women to be as complicated and contradictory in their personalities as we allow men to be, with their football teams and fishing rods. "Absolutely! Women are more complicated. Much more interesting." But what about the argument that our culture makes too much noise around women's looks as it is, and feminists shouldn't add their voice to this? "That's a quite separate issue from fashion," she says in that familiar strict Scots tone. "The emphasis on how you look is very narrowly defined in our media. That's the problem. Whereas women can look great in all sorts of different ways. And women on their own terms understand that. Women who are interested in fashion dress much more for women than for men, and with a complex idea of what is stylish which most men just don't understand." I could hug her.

Does power dressing come into her wardrobe choices? "It used to. When I was first in front of the camera, a suit was a suit of armour against the world, to give gravitas. But as you get older, that gravitas comes from within. I remember when I interviewed Mrs Thatcher [in 1990], thinking quite carefully about what I wore: I wanted to look clean, and sharp, so I wore an Armani jacket. Now, it's more a case that there are certain interviews for which you want to choose an outfit you feel confident about and then not think about it again."

After our shoot, Wark is heading straight for the 2pm meeting for that night's Newsnight. Tonight, she's wearing a Prada skirt with an Etro shirt, "which I'd looked at before but didn't buy until I found it for £90 in the Liberty clearance room. I'm a bargain-hunter – of course I am. You don't get a clothing allowance at the BBC, after all. I love Topshop. But things change as you get older. When you're young, you can get away with throwaway clothes, but these days the line has to be better, the fabric has to be better. And I've learned that certain labels and pieces work for me." Prada skirts and Agnès B tops are staples that don't date, or let her down.

On the shoot Wark is warm, easy company, good fun. The only glimpse of her steely side comes when I ask how someone whose hobby is baking – she reached the final of Celebrity MasterChef – stays a size eight. Such a crass question, I know, but I was hoping there was a secret. Turns out there isn't: "Baking is relaxing. I make bread, but I don't eat it myself. I make homemade pasta, but I don't eat it any more. When you get older, you have to be a bit more careful. And I exercise quite a bit – I play tennis every Sunday morning with a friend. Even this weekend, when it was snowing." I must look a bit shocked, because she quickly adds, "But it wasn't a blizzard or anything. Just a flutter, really."

Wark has recently finished her first novel, set on the Isle of Arran. "It's a twin narrative, the story of a 96-year-old woman who has just died and of the young woman who comes to live in her house in strange circumstances. And there's a secret." It's a novel about women's lives and the pull of the land – something Wark, who travels back to Glasgow on the sleeper when she's done at the BBC for the week, feels strongly about. "Older women often have the most extraordinary stories, whether to do with family, or the war, or whatever. But we tend not to notice women beyond a certain age. I wanted to write a story that questioned that, just a little bit."