It is pointed out to him that you cannot ban over-fat people. "No. But you can make it that society does not accept over-fatness as a normal, healthy thing to be - just like society accepts that it's not good to smoke. We've created an environment where smoking is not only accepted as being a major health hazard but also one that is anti-social.

OF COURSE it's slightly more complex than that - the big difference being that nobody needs to smoke, but everyone needs to eat. Birkbeck (who was once medical director for WeightWatchers) says what he's been saying for decades: we need to eat less and exercise more.

"Of course, the one way to solve the whole problem is, in a dictatorship, you say 'Everybody that comes back in a year's time with a BMI of more than 30 will be shot' - and you'll find hardly anyone has a BMI over 30.

Let us assume that society does decide to extend its intolerance of nicotine to obesity: what then?