'You see, I don't want that watermelon. But if I did, that would stop it going in to landfill." Simon Barnes, 47, and I are standing by the reduced aisle in a Tesco store in Kennington, south London. In fact, I do want the watermelon. However, I feel that it would be infra dig to conduct this interview carrying a watermelon.

This was the week in which Tesco revealed that it had binned 30,000 tonnes of food waste in the first half of 2013. Like holding their nose and jumping into a cold pool, Tesco bosses decided that the transparency was worth the opprobrium, which I think will turn out to be true. The main thing that struck a chord was not the profligacy of supermarkets but the elegiac decay of the bagged salad: more than two-thirds of it thrown out, half by customers, half by stores. So much waste, so little actual ingestion. It's like a leafy metaphor for the good intention.

Picturing Tesco's statistic, however, one imagines heaps of food out back, baked goods piled high on maroon plastic crates, a totem of pointless waste. That's just not how it works any more. The total waste along the food value chain (this is how they talk, OK?) is 32%; 16% is wasted in "agriculture and supply"; 16% by the consumer. The amount wasted by retailers is, they say, less than 1%.

Shoppers could and do argue that their waste is spurred by supermarket offers; farmers could and do argue that their waste is caused by factors outside their control, such as: "I cannot eat this parsnip because it looks like the gnarly hand of a wizard."

The fact remains that, when you go into a Tesco superstore, the operation is as lean as a hare. "People come in at around half eight, because that's when things start to get really cheap," Barbara, the deputy store manager in Kennington, explains. "We make an announcement on the PA and people will be here immediately."

Nigel Leigh, 58, is a one-time university lecturer and documentary maker. He arrives from nowhere, like Nanny McPhee. A minute ago, there was only a fruit salad, a watermelon, and some pre-cooked rice noodles, only modestly reduced from £1.20 to 71p. Suddenly, there are three ready meals, one of which is a mushroom risotto for 36p, 10% of its original price, and Nigel has it. His manner is that of a mischievous soothsayer.

How did he know? There was no announcement. "I'm the guy you're looking for. There's a certain kind of bottom feeding shopper. And I'm it," Nigel explains.

There are people who will buy things simply because they're on the reduced shelf, before they have been marked down still further. Those people are stupid, thinks Nigel. "But you can make a mistake the other way. You can see something that you actually really want, but you know it's not as reduced as it could be. So you leave it for a while. But guess what – there are other people in this store as well, and they will buy it. You can't let yourself go home miserable if you've only got a discount of 50%."

Nigel does, inevitably, have rivals. "There's a guy who comes in, I think he's a Korean student. If I see him, I may as well go home. He'll have the lot." And there are other people who don't abide by the code of honour. "They go to the shelf, put everything in their cart, and then, like an animal, they'll take it away and look at it somewhere else. Bring back the stuff they've decided they don't want."

But if there's competition, there's no ill-will. "Me and them, the guy who supplies the shelf, the guy who takes it off the shelf, we're all victims of a world economy over which we have very little control."

There's a tub of clotted cream sitting like an orphan on the shelf. "The important thing is," Simon underlines by picking it up and waving it about, "never to buy things that you're not going to eat."

Earlier Svetlana, 29, made a similar point. "I am always throwing away cream."

I catch up with Nigel again near the dried goods. "I've got wealthy friends with big houses in Barnes, they love me to regale them with tales from the reduce-to-clear life. Their wives think it's there-but-for-the-grace-of-God-go-I. But I think it's a little taste of the Serengeti. It's a system that rewards the quick and the brave. The decisive. You need to know what you want, know where it is, and take it.

"I'm just playing around with capitalism's artificial pricing system. I will get it at my price. David can win against Goliath, but only if he comes in after eight o'clock."