This is a lesson that the once-mighty Sainsbury's has learnt to its cost. Last month it announced the worst pre-tax results in its 139-year history - a first-half loss of £39 million, its first ever loss - largely as a consequence of poorly stocked shelves and a failing supply chain. It has been a long, hard fall for the store, which was the UK's leading supermarket as recently as the mid-1990s.