A few years ago, while Jane Fearnley-Whittingstall was changing the nappy of one of her four grandsons, he tumbled off the bed on to the floor. “I won’t tell you which one because I haven’t yet confessed to the parents,” she says mischievously. The key to negotiating the politics of family relations, Jane believes, is knowing when to hold your tongue and when to brace yourself for confrontation. “I err on the side of avoiding rows or shouting, but I hope I’m not a sweeper-under-the-carpet person.”

I bet that, mostly, her judgment is spot on. A friend of mine with a newborn baby, who knows Jane well, says: “I adore her; I’ve adopted her as my mum.” Indeed, at 66, with her trim figure, funky but age-appropriate Jigsaw clothes, her mixture of traditional values and modern savvy, Jane is the very picture of the 21st-century granny. Which is no surprise since her book The Good Granny Guide (Short Books, £12.99) has been an unlikely bestseller. This is a fine repository of tips for those who find themselves pushing a pram for the first time in 30-odd years: from how to stop toddlers cutting their throats by charging into your drinks cabinet, to how best to make pleasing paper chains.

But it also bears the mark of Jane’s slightly vinegary family wisdom: “Don’t confide in your daughter or daughter-in-law that it makes you shudder to see a two-year-old with a dummy.” And if your grandkids won’t eat healthy organic food well, who cares? Leave that battle to their parents — and bring on the ice-cream. “I get a special pleasure,” she says, “from buying white bread for my grandsons because it’s forbidden to them at home.”

And now Jane is to turn her sharp eye to a Body&Soul column about modern family archetypes, starting with the Little Emperor (see right), and followed next week by the Over-Sexy Sister-in-Law. She won’t just be lampooning the younger generation. “Some old people drive me mad. They see the past with rose-tinted glasses and refuse to try anything new,” she says.

In an age when we are constantly being bossed around by TV experts — nutritionists, estate agents and “stylists” — Jane represents the triumph of experience over expertise. “I come from a primeval tradition of knowledge being passed from one generation to another by the elders,” she says. So does age truly bring wisdom? She grins: “Of course, I think I’m terribly wise.

But it doesn’t apply to everyone. Many of my contemporaries are as feckless as ever.”

Before becoming the nation’s head granny, Jane had already achieved a degree of fame for two reasons: producing seven books on gardening — Peonies: The Imperial Flower (Weidenfeld & Nicolson, £30) is highly regarded — and for giving birth to Hugh Fearnley-Whittingstall, the TV chef of River Cottage fame. Indeed her whole family seems to write: her husband Robert is a creative director in advertising, still copywriting aged 70, and her daughter Sophy is a financial journalist.

Each of her children now has a brace of young sons, and Hugh and his French wife Marie are also raising a nine-year-old Congalese girl Chloe, whose mother died when she was a baby. So does Jane think that modern parents are terrible spoilers? “I spoilt my own children,” she admits. “They’d demand my attention at a party and I’d let them talk, which must have been very unenchanting for other adults.”

Her main grievance against modern methods is that children are allowed to encroach on adult fun. “You can’t sit down with a nice glass of wine, if you have grandchildren to stay, until about 8.15pm. My own children had lights out at 7pm, but I suppose modern parents work late and otherwise wouldn’t see their children.” She is utterly uncomprehending — like most older people I know — of today’s children being allowed to sleep in the parental bed. And she believes that TVs and computers in children’s rooms limit their interpersonal contact, thus creating a generation of grunting social inadequates.

And she is white-hot on manners. “If I don’t get a ‘thank you’ I leave a polite pause for the child’s parent to demand it. Then, if they don’t, I will say something.” Yobbery, she believes, is born when a parent fails to demand such respect: “People are frightened of their children, of saying the wrong thing.”

She recalls sitting on a train recently, surrounded by a group of young soldiers who were quite boisterous and drunk. “One of them opened a can of beer and it sprayed all over me,” she says. “I know I should have found them intimidating, but I thought: ‘They are only just like Hugh and his friends when he was their age, a bit rowdy and keen to have a good time.’ So I tore the soldiers off a strip. And, do you know, they were very apologetic and nice after that. It’s just tha people won’t criticise bad behaviour in public like they used to.”

Many modern grannies — with active lives, still full of travel and even work — often complain that they have their grandchildren dumped on them by children eager for a break. But Jane yearns for their visits to her house in Wottonunder-Edge in Gloucestershire, with butterflies in her tummy. “It feels so much like being in love when I was younger,” she says. “The tingling anticipation of seeing your beloved.”

And she is glad to help out. She remembers being a young mother herself, “that zombie-like phase when I didn’t comb my hair for three years”, and the loneliness of being with toddlers at home all day. Her own mother-in-law was a saviour: “My husband and I would dump two kids and a dog on her for a whole week and go off on holiday. We just took it for granted.”

Although she is at a time in her life when some people begin to wind down, Jane is busier than ever. Besides her new column, she and Sophy have set up a website www.goodgranny.com. “It’s lovely, but I’m a bit worried about when I’ll have time to see my grandchildren.”

ONLINE Q&A

Visit www.timesonline.co.uk/talkingpoint to e-mail your questions to Jane Fearnley- Whittingstall. She will answer on online on Thursday, December 1.