She’s a consummate TV performer, writer of more than 70 cookbooks and her show has just won a Bafta. But to millions, Mary Berry – whose name is a winning concoction of unthreatening Englishness and healthy nutrition – represents not so much a media star as a comforting kitchen presence. Above all, she is a brilliant communicator of wholly reliable recipes. There is no worry that a Mary Berry cake won’t rise, her puddings won’t set or her roasts won’t crackle. If you follow her instructions, the food comes out right.

I first came across her about 18 years ago when I had moved to a house with an Aga. Agas, you see, are a Mary Berry standard. In those days, she ran regular Aga Cooking Classes, so I signed up for one, in order to learn how to use the damn thing. At Mary’s apron strings, it took just a day for me to grasp every single important thing about an Aga. I even learnt how to iron with it. I also learnt how to relax around food, how to enjoy cooking and – crucially – how to make pastry. She was a formidable force then, but now, fronting The Great British Bake Off, she has moved into a different zone of media saturation.

This is not to say Mary, 77, has transformed into some horrendously needy media queen. She still has that gentle air and sweet, slightly tolerant smile that you suspect has probably carried her through many occasions that might have felled more irascible cheffy types.

She’s also amazingly slim, which is impressive once you acknowledge that working on The Great British Bake Off must involve consuming zillions of calories. “I’m just thrilled that it’s such a success,” she says, charmingly when I go to meet her on set. “Now we are in the third series, more people have entered it and the standard is very high. They think above the norm.”

That doesn’t mean that Mary doesn’t engage with innovative recipes, but getting people to concoct snail porridge, or treat a kitchen as a zone for pan hurling, or a laboratory for ice–cream fission, is just not her style.

Since her first cookbook was published in 1966 (The Hamlyn All Colour Cookbook), her mission has been simple. “To encourage people to cook, to bake and enjoy it,” she says. Television? No problem. “I think television is like a huge cookery class. That’s the joy of the masterclass element in this show. Once viewers have seen people attempt something like a Battenberg cake, and get into trouble, then I come along and slowly and carefully show them how to make the perfect Battenberg, stopping when it might go wrong and encouraging them to have a go at home.”

Home, and the straightforward pleasures therein (rather than sex, travel, swearing, weeping or any other of the current hooks for television cooking shows), is at the centre of the Mary Berry vision. “I do family recipes for different occasions. I don’t use too many different ingredients,” she smiles. “I don’t use ingredients that people aren’t going to use again. And I don’t make things complicated.”

A typical recipe will have no long list of different tasks, no weird spices, and no requirements for bizarre utensils such as octagonal saucepans. Is it any wonder her books are so popular?

“I am the first person to go out and buy puff pastry made from butter,” she says. “I won’t make things that are going to take an awful long time, because people have other things to do with their time. You have family life, don’t you?”

This is a somewhat bittersweet observation, since Mary and her husband, Paul, were bereaved 20 years ago when William, one of their three children, died in a car crash. “We were lucky to have him. And if he walked through the door now I would say, ‘Where have you been, young man? Come on,’” she says briskly.

The loss is still clearly felt today. “It doesn’t go. And we miss him enormously. As a family [she has a daughter, Annabel, and a son, Thomas] we are forever talking about Will. We remember the good and the bad, he was naughty just like the rest of the children, at times. But every family has something that happens in their lives.”

Languishing is not something that Mary does. Looking on the positive side, even amid the horror of losing a child, is the MB way. “I am really pleased that if he was to die, that he didn’t leave a wife and children,” she says. “Because you know, that would have been a sadness for them. And he died at a time when he was truly successful with his sport and his school, and he was totally happy with himself. So we have really nice memories of him. He had done well.”

It takes fortitude, being able to say that about your son of just 19 years, who died at the wheel on his way to pick up a paper at the village shop. And it’s what seems to have sustained and pushed her forward through an impressively long career.

“I like to be busy. But after Will died, I didn’t want to continue working in London. I wanted to be at home with my husband. So that is when I set up the Aga Cookschool. And that didn’t half fill my mind. Fortunately it was very successful and I loved every minute of it.”

I realise this must have been around the time I met her. I remember her, appearing so happy and proactive, at home in her kitchen, demonstrating how to dry herbs in her beloved Aga, instructing us to keep the back door open so that dogs (if you had them) could trot in and out.

The importance of family to her connects perfectly with her style of cuisine. “We are now going back to home cooking, particularly in this recession, and I am encouraging it,” she says. “I’d like to see more sitting around the table at least once or twice a week. It can be the only time you get together with your family. By the time children have had their meal, even the monosyllabic ones will want to chat."

It is a particular joy that The Great British Bake Off and its spin-off Junior Bake Off have encouraged mums and dads to cook with children. She always cooked with hers and now cooks with her five grandchildren. “I hear that children are asking, in that little bit of time they have before tea, after they get home from school – shall we make a biscuit? Shall we make a scone?”

“When I did Desert Island Discs, Kirsty Young came in with a bag of cookies that her nine-year–old daughter Freya had made. Freya wanted to know what I thought of them. Everybody is setting to, and baking,” says Mary happily, who says she gets about two emails a day from schools, offices and clubs – what she calls “the bake off fraternity” – telling her about their scones. And countless emailed pictures of people standing behind “a tarte citron, or a dish of brandy snaps.”

Any complaints? “Oh yes, I get emails from people saying I tried such and such a recipe and my cake went down in the middle.” A tinkling laugh. “They never think it might be them, or their oven. I mean, every oven is different, isn’t it?” she says in that lovely comforting, positive manner that is so very Berry.