“You won't be surprised to hear it's very opinionated,” says Esther Rantzen as she hands over a copy of her new book, If Not Now, When? And I'm not: on television and radio Rantzen usually comes across as a bit of a know-all, so that even when she is right - as she so often is, damn it - you can't help feeling a bit irritated.

Now the ChildLine president and erstwhile queen of campaigning TV has turned her zeal to improving the lot of the over-fifties, by supporting a new government-backed website, www.fiftyforward.co.uk, which offers advice on everything from learning and spirituality to health and relationships.

Her new book is her upbeat take on “living the baby boomer adventure”. She explains: “I think the website is the Government saying, ‘Let's not assume that this growing sector of the population is a drain on resources; we should regard them as an asset and encourage them to have confidence to explore their skills and talents'. The great advantage of the third age is that you have the time to nurture the neglected bits of your life.”

At 68, Rantzen is ruthlessly selective about how she spends her time: not on chores, that's for sure. She enjoys work and remains a familiar face on television 14 years after the consumer affairs programme That's Life! ended its 21-year run, during which it regularly attracted more than 18 million viewers. She recently trained as a ChildLine counsellor and is about to take courses in cooking and dance exercise. Nevertheless, life after 60 has been tough. Rantzen's husband, the documentary maker Desmond Wilcox, died eight years ago and soon afterwards their three children flew the nest. A more vulnerable side seems to have been exposed: Rantzen is frank about the downsides of always seeming so right about things, and even claims to be working on humility.

She has recently stopped dyeing her hair

One upside is that she has grown into a new ease with her looks: while she is a huge fan of Botox, she recently stopped dyeing her hair after 40 years. “I just thought, enough already, enough,” she says. “I had no idea what colour my hair would be underneath, so I just let it grow out. It was part of a process of getting back to see who the hell I am now. I'm lucky in having inherited my father's hair; he never went grey.”

The soft-brown barnet is a pleasant surprise after Rantzen's flirtation with red (“not an easy colour”) in her Strictly Come Dancing phase. Mercifully, she also appears to have ditched the racier end of her wardrobe, along with the yellow convertible Beetle she gadded about in for her most recent investigative show, Old Dogs, New Tricks, which she co-presented in 2006 with Lynn Faulds Wood. You could never accuse her of not being game. But even in the comfort of her Hampstead kitchen, which is painstakingly neat apart from a pile of her son's clothes, Rantzen looks every inch the well-groomed TV legend, in matching pinks and big pearls.

The morning we meet she is bombarded with requests to appear on Richard & Judy, LBC et al after her Daily Mail column appeared to condemn ChildLine as a “politically correct monster” - she says the headline was misleading. She needs reassurance, but evidently thrives on the attention and the added pressure.

In conversation she is witty and surprisingly self-deprecating, and her trademark raised eyebrow and complicit smile are largely absent. One quickly realises that it's her didactic screen manner, rather than what she says, that raises hackles. In her defence she says: “I sound very certain in my views; actually they very much depend on what I hear and see. But then I have been trained to express myself cogently and I'm also nervous of boring people. It's not very loveable. My late husband used to say, ‘Could you not preface what you say with, ‘Well, I think' or ‘I wonder whether it's true that' and I would think, I suppose I should, but it takes so much time. I'm working on humility; that's another new thing I'm doing.”

When I inquire how she plans to acquire it, she laughs: “About the same way that I'm working on exercise - I'm thinking about it! I may never get there. I hope there will be a humility course on the Fifty Forward website. But I'm already learning it from my garden in the New Forest, where I have a country cottage. The plants say, ‘Don't think you can dictate to us; we will thrive where we want to thrive'.”

Rantzen says she inherited “a kind of disobedience” from her mother, and her rigour from her odd-sock-wearing father, who was head of the BBC's Engineering Designs Department. She re-created the noisy household in which she grew up for her three children. So how have they coped with her certainty? “They are aware, that it's eggshell thin,” she replies. “They know all about the soft bits behind the bold front. So they look after me; they're terribly supportive.”

The soft bits show when we meet. The night before she had appeared on a chat show with the Iranian comedian Omid Djalili, and she is fretting that she came on a bit strong. “In my normal puppy-like way, I told him how much I admired him,” she winces. “I was a bit hyper because my daughter was having an operation and I was trying to block it out of my head, so I may have been over-enthusiastic. I've never seen a more terrified man! I think he thought he was sitting next to a rampaging 68-year-old.”

It is this kind of reaction that puts Rantzen off real-life dating as opposed to reality TV dating, which she tried four years ago in the BBC Two programme Would Like to Meet. She says: “I think I'm reluctant because my pride tells me that I am unlikely to be the object of anyone's ardent desire this late in life. So stick to safe territory, that's what I'm likely to do. Although as Cilla Black put it, ‘Never say never'.”

In any case, she recognises that the “loving, all-embracing friendship” she shared with Wilcox may be impossible to follow. The couple married for a second time after his conversion to Judaism shortly before his death, which has done nothing to shake Rantzen's Jewish agnosticism. “For me, life without Desi gets fractionally worse,” she says. “I was in his wake for a long time after he died, and I carried on in the direction we had agreed together. So now when I recognise that in September it will be eight years, that's quite tough. I feel his presence a lot; I think that's because I'm so imprinted with everything he was. My cousin, who is a psychiatrist, says there will come a time when I don't talk about him every day, but I don't know.”

Since his death, Rantzen has added Wilcox's “OCD habits”, such as obsessively shutting cupboard doors and drawers, to her own: she can't pass a bin lid without closing it, or a cushion without plumping it. She is still full of regret for missed opportunities, such as the round-Britain road trip they had planned, and anger that he died too young to witness their children's achievements. Emily, 30, works for a children's charity, Rebecca is a television reporter and presenter (Rantzen prefers not to give her age because she believes ageism can affect even the young), while Joshua, 26, is at medical school, taking his second degree.

Emily has recovered from the chronic fatigue syndrome that forced her to leave school after GCSEs and lasted 14 years. At her worst, she spent 24 hours a day in bed and needed a wheelchair and a stairlift. But her health improved rapidly two years ago when she embarked on the Lightning Process (www.lightningprocess.com), a training programme for mind and body based on the theory that ME affects the body's capacity to deal with adrenalin.

“Healthy? It all seems to be functioning”

When asked whether she is healthy herself, Rantzen replies: “I don't know, sweetheart, I don't know. I don't ask myself too many questions on the subject. Every now and then my GP says let's have a look at this or that, and it all seems to be functioning. I don't drink a lot because alcohol makes me drunk very quickly, and I gave up smoking in my thirties because an aunt died of lung cancer.”

Rantzen appears to have overcome much of the insecurity she once felt about her appearance, shrugging off unkind remarks about her prominent teeth. When she was younger she went on “ludicrous” diets because she was so unhappy about her weight, until WeightWatchers taught her about calories and she developed a taste for undressed salad. She's thin now, with no conscious effort, perhaps, she suggests, because her tastes have refined with age.

Would she have cosmetic surgery? “Yes, defnitely. I've done Botox. It's like a hornet stinging you, but it's worth it because you get that lovely serene forehead. I'm not discussing my other flaws because once you do that's all anybody talks about. Even after my teeth were fixed over a period of years, people still referred to me as toothy Esther Rantzen and drew cartoons with the old teeth. But you just laugh; they've got to have a label, haven't they?”

If Not Now, When? Living the Baby Boomer Adventure (Headline Springboard, £16.99)

www.fiftyforward.co.uk is the new interactive website developed by Learndirect

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