She's observed from afar the decades of endless weight-loss fads among her famous rivals - no doubt, with something of a self-satisfied smile.

Because Jane Seymour - who at 74, was born in the postwar rationing era - has had neither the need nor desire to experiment with anything remotely resembling a dieting 'trend'.

And yet she's still precisely the same size and weight she was 52 years ago - size 8 and 8st - when she was a Bond Girl alongside Roger Moore in Live And Let Die.

Her tricks to maintaining her age-defyingly lithe figure rely on titanium self-discipline.

From exercising in a metal cage - called the 'TrueStretch', it's said to aid maximum flexibility and looks like something from a sado-masochist's dungeon - to not eating for 16 hours a day, Jane's willpower is, evidently, as rock solid as her abs.

These are particularly impressive when you consider that as well as being in her eighth decade she has also borne four children - twins among them.

So what, I wonder, when she speaks to me from her home in Malibu, after all these years of torturous self-control, does she think of all those gleeful celebrities who are now achieving a figure as lean as hers by doing nothing more arduous than giving themselves an injection - and microdosing with Mounjaro?

'I don't criticise anyone for anything they want to do. But I have no reason or need, nor have I ever had any desire to do anything like that,' she says.

Jane Seymour was a Bond Girl alongside Roger Moore in the film Live And Let Die

She walked the runway at New York fashion week for the very first time - just days before her 74th birthday - in a sexy, sleeveless mini dress

Vials of semaglutide weight-loss jabs are, then, clearly not tucked in Jane's fridge. So what on earth is?

She insists she denies herself nothing but then admits she touches no food until the afternoon.

'I try not to eat in the morning. I jump start very naughtily with black coffee. I'll also have plenty of water and special vitamin powder and collagen powder that I'll put into water and drink.'

And that's it until 1pm - which given that Jane is regularly up at 5am to record the latest series of her hit detective show Harry Wild - takes discipline to a whole new level.

Jane effectively follows the 16:8 diet, fasting for 16 hours of the day and compressing all her eating into just eight, a programme advocated by many experts as being the key to good health.

This is the theme that emerges as Jane continues to describe her regime: in a world where everyone can be slim thanks to fat jabs, she's instead in pursuit of that ever more elusive and aspirational target - youthfulness, inside and out.

Indeed, as the Mail's own Literary Editor Sandra Parsons wrote last week, new scientific tests measuring powerful biomarkers of chronic inflammation and ageing can determine your body's biological age. Through tweaks to her lifestyle - including a form of fasting, which means your body regenerates cells more efficiently when you do eat - she was able to keep herself 20 'on the inside', at the age of 61.

While Jane has never taken such a test, she tells me, 'my regular doctor told me that health-wise I'm maybe 15 years younger than I actually am,' which would make her around 59.

Jane Seymour with four of her children attending a charity event together in 2019

And so, with that in mind, lunchtime is when she says she eats 'a massive meal'.

Jane's definition of 'massive' is perhaps not everyone's 'massive', however.

'Usually I'll have shrimp, or salmon with a little olive oil and ginger, and I'll have lettuce and greens from my garden, as I grow everything organically.

'My favourite is cucumber salad which I make with Persian cucumbers and rice wine vinegar.

'From time to time, I'll have some lean chicken,' she adds, 'but I don't weigh anything. I just look at it to see if it's how much I want to eat.

'I almost never eat red meat unless someone serves it at their home and I stay away from fish that are larger than a salmon.

'If I want a carbohydrate, it'll probably be lentils or couscous. Or a really good pasta - either fresh pasta or a really good quality Italian brand.'

For dessert, Jane will usually eat raspberries, blueberries or strawberries - 'we grow a lot of them in the garden' - although rumour has it that on the rare occasions that she eats Maltesers, she limits herself to just four at a time.

Jane's tricks to maintaining her age-defyingly lithe figure rely on titanium self-discipline

'No, not true,' she laughs. 'But I do love a piece of Lindt 70 per cent dark chocolate. It also happens to be very good for you because it's full of antioxidants.'

Evening meals are often skipped: 'I'm just quite happy with a very light snack, or nothing at all.'

Snacks include pistachios and almonds - and if she's truly desperate for something a little naughtier, 'sometimes to [appease] my craving for crunchy, salty things, I'll slice sweet potatoes really, really thin, dry them off, sprinkle a little sea salt and olive oil on them and bake them very carefully in the oven, making sure they don't burn. Those are my idea of [crisps], but healthy ones.'

Alcohol is similarly minimal: 'I almost don't drink, but if I do have alcohol, I have just a very small amount - usually a glass of wine, but certainly not every day and when I'm filming, not at all.'

It all means she looks extraordinary on the outside and also has the vitality to keep in constant work.

Indeed, the Dr Quinn, Medicine Woman actress once said that despite being 'pregnant with twins and pre-eclampsia… I've never had a sick day in my life. They'll have to pull me off on a stretcher if they want to get rid of me.'

She also looked jaw-dropping earlier this year when she walked the runway at New York fashion week for the very first time - just days before her 74th birthday - in a sexy, sleeveless mini dress.

Her Instagram account reveals similarly enviable images to her 400,000 followers: doing ballet plies in a low-cut blue swimsuit in her garden and confidently showing her cleavage while chatting to her followers in the back of a taxi.

And just as her dietary discipline commences from the moment she wakes, so too does her exercise regime, after a good night's sleep.

'I make myself sleep eight hours,' she says. 'If I'm filming or working I literally set my alarm the minute I know I have to be up and make sure that I'm in bed attempting to sleep at least eight hours before'.

Then as soon as she's conscious she's previously admitted to following a dance teacher's advice to 'wiggle my whole body from side to side like a fish. It makes all the difference. Then I'll stretch in a hot shower, that really helps.'

Peloton exercise bikes are a no go - bad for the posture, she's said, because of that hunched position - but Reformer Pilates is very much in.

Practised on a moving bed with various straps, springs and weights for resistance, it's been part of her routine for decades.

Planks are also favoured - forearms on the ground, feet hip width apart, core pulled as tight as she can bear.

'And for the back of my arms, I'll turn my back to a table or counter and use my body weight to go up and down. Sometimes, I'll just lean against a banister and do press ups while standing.'

Nothing she does, she insists, needs a gym (though a reformer bed could set you back upwards of £3,000).

And everything is combined with endless walking on the sandy beaches near her house, and using 'one of those elastic [resistance] bands that I take with me wherever I go and put it around my legs to work my inner and outer thighs'.

'I don't call that a major workout,' she adds. 'I call that waking up my body and trying to get it moving. I work out carefully because I don't want to end up having to have hip or knee replacements, which a lot of my friends are having.'

Jane, daughter of an obstetrician who grew up in Merton Park, South London, may have lived in California for many years, but still retains a certain no-nonsense Britishness. Fillers are a no go, as is Botox.

'I tried Botox a couple of times and I just thought, it doesn't work for me. I don't have anything against anyone doing it and it's a great product if I wanted to have my face not move. But I'm an actress and I'm paid highly to move my face.

'A million years ago, I did [have surgery] under my eyes, but I wouldn't do that again,' she says. 'I have a muscle - it's not a bag - under my eye and I've just learned to live with it.'

She once said that if someone wanted to look younger, it's 'super-easy': put a wig on, it pulls your hairline up and, hey-presto, a naturally 'lifted' face.

Lines on a face are a sign, she adds, 'of a life well-lived. But sometimes I look in the mirror and go, “Arrrgh!“ And then I realise there's nothing more beautiful in a human being than somebody who's happy.'

The four-times married Jane is certainly happy with her current partner, musician John Zambetti, 75, whom she met on a blind date aged 73.

'Loving and being loved definitely helps,' she says.

There's clearly a zing to their relationship - she told Cosmopolitan Magazine recently that 'sex right now is more wonderful and passionate than anything I ever remember because it is built on trust, love, and experience.'

Words, clearly, that come from the heart because her four husbands, she once noted, all left her for other women.

Her Instagram account reveals similarly enviable images, which she shares to her 400,000 followers, such as of her doing ballet plies in a low-cut blue swimsuit in her garden

Her first marriage at 20 to theatre director Michael Attenborough (son of Richard) ended after two years, while her second to Geoffrey Planer, the older brother of The Young Ones star, Nigel Planer, lasted barely a year.

Her next to businessman David Flynn produced two children - Katherine, now 43, and Sean, 39 - but ended in disaster when Flynn lost her fortune, leaving her in debt to the tune of around £6.7 million.

Securing the title role in the hit 1993 series Dr Quinn, Medicine Woman saved her, she later remarked, from being 'penniless and homeless'.

Her fourth marriage to actor and director James Keach produced twin boys John and Kristopher, now 29, though that too ended in 2015 after 22 years, when she discovered he had been cheating.

Remarkably, Jane managed to stay friends with all four exes.

Has a forgiving nature perhaps contributed to her less harried, more youthful countenance?

'I think living with gratitude and in the present... makes you feel good about yourself,' she says, adding: 'Things happen to everyone. You move through it and you move on and you learn from it. And I actually value all the changes that have happened in my life.'

But throughout all those changes, it's truly extraordinary that Jane has remained almost entirely the same - long-haired, gleaming, and, resolutely, that perfect size 8.

Harry Wild Season 4 is out now and available to stream on Acorn TV, Amazon and Apple.