They're not talking about me, are they, in that fatty campaign thingy, the one done by the Wallace & Gromit people? I'm not obese. This new government weight campaign, the one with the Stone Age people modernising and growing flabby, is for the fatties, isn't it, and we all know who they are. It's not going to work either, is it? Because the very people the campaign is aimed at will ignore it, won't they?

Well, yes, probably. Because the people it is aimed at really is you and me. Public-health campaigns such as Change4Life, launched last week, have the greatest effect if a large number of low-risk people change their behaviour; far greater than if the smaller number of high-risk people do. So, yes, it is you and me they are talking to.

That brings its own problems: while the benefit to society as a whole if lots of low-risk people eat slightly better is large in terms of savings for the NHS in future, the benefit to the individual is small. Which is why nearly all public health campaigns fail; and why I suspect that this one, all £75 million of it, will as well.

A man with the marvellous title of Winton Professor of the Public Understanding of Risk, David Spiegelhalter, of Cambridge University, last year analysed for the Royal Statistical Society the effects of a campaign to reduce alcohol consumption. He showed that a 20-year-old man drinking a “hazardous” four units a day who reduced his intake to the recommended safe limit of one per day, will gain 73 extra days of life, or 20 seconds for each pint not drunk - which may seem a poor return for forgoing the pleasure. While ministers and public health officials give advice based on what is good for society, Professor Spiegelhalter concluded, “individuals receiving that advice may, equally reasonably, choose to ignore it”.

We're tricky like that, we are - and we hate being told what to do by ministers. As the Government admitted four years ago, after a mammoth consultation exercise to decide what to put in a White Paper on public health, the overwhelming message was: go away.

“First, people told us that they want to take responsibility for their own health”, wrote John Reid, the Health Secretary at the time, after consulting 150,000 people in one way and another. Mr Reid was never a big fan of the bossy government agenda: “They were clear that many choices they made - such as what to eat or drink, whether to smoke, whether to have sex and what contraception to use - were very personal issues. People do not want government, or anyone else, to make these decisions for them.”

Those were the days. Hark at Alan Johnson, the present Health Secretary, last week: “If we do nothing, by 2050 we could be living in a Britain where two thirds of men and half of all women are clinically obese... That is why we are doing something about it NOW.”

The reason for the change (4Life) is that ministers have realised - and this happens every few years or so, hence the endless campaigns - that if the Government doesn't actively do something about public health, nobody else does either. The question is, when ministers do try to do something about it, do we listen?

For decades, doctors have been warning of an obesity “time bomb” and ministers have been launching healthy-eating campaigns - remember Virginia Bottomley's “three egg-sized potatoes a day”?

Under Labour alone there has been a White Paper on “healthy behaviours”, a “food and health action plan”, the “5 a day” fruit and veg campaign, a “choosing health” White Paper and last year, “Healthy Weight, Healthy Lives; A Cross-Government Strategy”.

Result: a great, fat zero. The nation's waistline has swelled with the list of public-health campaigns. When Mrs Bottomley was Health Secretary 13 per cent of men were obese and 15 per cent of women; in 2007 that had risen to 25 per cent of all adults. A third of children today are overweight or obese by the age of 11. It's our lifestyle: too many calories in, not enough exercise to burn them off. If present trends continue, by 2050, ministers expect levels of obesity to rise to 60 per cent in men, 50 per cent in women and 25 per cent in children, costing the economy £50 billion a year.

If we cannot improve our public health, the NHS will become unaffordable. Simple as that.

So, what to do? Health ministers claim to be trying something completely new: not just a public health campaign but “a lifestyle revolution”, no less, in Mr Johnson's words. Having persuaded food and supermarket companies to sign up, they plan to use “social marketing” techniques to penetrate the public consciousness.

Don't know what a social marketing technique is? Here's the scary thing - nor do they! If I could refer you to the website of National Social Marketing Centre - www.nsms.org.uk - you will see that the whole idea gets really boggy. The National Social Marketing Centre, you will learn, is a “strategic partnership” between the former National Consumer Council (NCC) and the Department of Health, set up two years ago after a report into the nation's health commissioned by the NCC, which recommended greater use of health-related social marketing, which is “the systematic application of marketing concepts and techniques, to achieve specific behavioural goals to improve health and reduce health inequalities”.

Which sounds to me like a public health campaign. And we know how effective those are...

When this campaign fails, and I hope it doesn't but I think that it will, ministers should be ready to legislate to force a change in behaviour. Smoking only dipped sharply when it was banned in public places.

Strict food labelling, sugar tax, treadmills... I don't know. The makers of Wallace & Gromit might be able to come up with an idea or two. Gromit doesn't have a mouth.

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