Do you know what raises Joanna Lumley above the herd? Apart from, obviously, her textbook poshness, classic swearing, and egret-like opalescence? She has one fag a day. One fag a day. Instead of chonging them down, recklessly, like some thoughtless, deadline beagle, Lumley tabs up a single bifter, and smokes slowly, with Nile-like calm. She has pondered on the best bits of being a smoker - Zeppelin-headrush; a moment of quiet reflection - and realised there's no need for that constant lighting and stubbing. All that addiction. You can just go in for the best bit - leaving all the cough- causing stuff to the more foolish.

It's an intelligent take on being a sybarite - the sort of thing you would expect from someone who looks as if she's made entirely of hand-cream, poetry and champagne.

For her services to thoughtful hedonism, Lumley equals the former President of the United States, Bill Clinton - a man whose many admirable qualities were eclipsed by his seminal pronouncement on chips. After Clinton suddenly lost four stone, he divulged his pivotal secret - Key Chip Eating. He hadn't given up eating chips (as in French fries), he explained. He'd simply narrowed down his chip eating to the first, and then the last - and dispensed with the usual 300 in between.

“The first and the last are the ones you enjoy the most,” Clinton explained, like a potent Newton, giving his first lecture on gravity.

It will be a sad day for fans of these admirable moderation theories, then, when Joanna Lumley: In the Land of the Northern Lights is broadcast. For despite limiting herself to a single cigarette a day, Lumley has found herself unable to limit herself to a single BBC travelogue “fulfilling a childhood dream”. We are now all the poorer as a result.

The dream fulfilment was fine. In 1994, Lumley spent nine days on a tropical island, “being Robinson Crusoe”, as a “fulfilment of an etc, etc”. For that show, she ended up making shoes out of her bra, and cadging cigarettes off the camera crew (but, obviously, only one a day), in a very jolly hour of television.

But in 2008, she's at it again, like an addict - this time, asking the BBC to take her “up the Arctic Circle”, to see the northern lights. Raised in Malaysia, as a young item made of hand-cream, poetry and champagne, Lumley never saw snow, or even frost. “Putting on a cardigan was a huge treat,” she says, with knowing disingenuousness.

So off she goes to see the sugar snowscapes, dinky villages and the quietly booming fjords of Norway; while we, theoretically, enjoy the subsequent feast of cooing posh bird and bonzer cinematography.

The only problem is that, this time around in having her childhood dreams fulfilled during a high-budget BBC travelogue, Joanna Lumley seems to have forgotten how to be Joanna Lumley.

Sure, she turns up at the railway station in a floor-length fake fur, and a battered, 50-year-old suitcase with “LUMLEY” stencilled on the side. And, equally obviously, when she opens the suitcase, the sum total of her preparation for the Arctic Circle appears to consist of an antique Paisley pashmina, a small set of oil paints, and an ancient book-map of Norway, entitled The Vikings.

But from thereon in, the winsomely imperious and silly side of Lumley seems oddly mislaid - perhaps disrupted by its proximity to the Magnetic North. Instead, Lumley transforms into an oddly simpering item - alternately kvetching, orating and flirting until, on occasion, she comes to resemble a slightly demented Bette Davis. The Ice Hotel is too cold - Lumley has to go to bed wearing a hat, which is “the worst thing I've ever done”. She makes sheep's eyes at ancient fishermen, breathing “I can't wait to see your village” at them like Monroe singing Happy Birthday to JFK. And when Lumley finally sees the northern lights - which are, admittedly, about as insane a sight as we'll see until Nasa offers mini-breaks to the Tannhäuser Gate - she turns into Sally Field at the Oscars.

“It feels like it knew how much we wanted to see it,” she sobs, before turning her face up to the sky, and husking “Thank you! Oh, thank you!”

It would have been far better if she'd just lit up a fag, eaten a very tiny chip sandwich, and said, crisply, “How lovely.”

Joanna Lumley: In the Land of the Northern Lights, Sun, BBC One, 9pm

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