The British supermarket chain Tesco has banned the wearing of pyjamas in their store in St Mellons, near Cardiff.

To those with a little local knowledge, this may seem like bad business. The St Mellons Tesco serves an estate in which unemployment was high even in the supposed boom – and where the inhabitants did not seem unduly upset by the fact. It is not a place where the wearing of nightwear in the day is unusual.

So Tesco’s ban might exclude a considerable segment of the local market from their shop.

Yet a little deeper local knowledge reveals an intelligent exercise in market positioning.

Although the name St Mellons is now associated mainly with the estate, St Mellons proper is a pleasant village immediately to the north. Properties in the estate use the name for marketing purposes. The affluent villagers do not like being associated with the estate: in a market positioning of their own, they have renamed the village Old St Mellons.

The St Mellons Tesco serves not only the estate but Old St Mellons and several other nearby communities as well. Although the residents of the estate are numerous, they are not the highest spenders.

So Tesco’s St Mellons strategy is to exclude lower spenders in order to make their shop more attractive to higher spenders.

This fits in with their national strategy. Britain’s famous obsession with class is largely a thing of the past, but a strange class system still prevails in where you go to the supermarket: the well-heeled can afford Marks and Spencer while the poorest go to Lidl. In between, Waitrose is seen as more prestigious than Sainsbury’s and Sainsbury’s ranks above Asda.

Tesco sees itself as ranking above Sainsbury’s, so the media coverage of its St Mellons decree is free advertising of its image as an upmarket retailer.

The irony is that there have been television adverts in recent years in which shopping in pyjamas has been portrayed as “cool”. This dates from the Coen brothers film the Big Lebowski, in which the slacker hero practically lives in his nightwear. It is another lesson of marketing that what looks cool in LA simply looks squalid in places like the St Mellons estate.