As a result, Britain has no need for an equivalent of the Aswan or Hoover dam. Nor does it have the kind of water crisis currently affecting Spain, where a thousand reservoirs are not enough to fill the country's taps and irrigation channels, and a national hydrological plan envisages building 200 more dams and a new national water grid at a cost of pounds 20bn.
British leaks are not exceptional. Even cities with drastic water shortages, such as Amman in Jordan, leak an estimated 40 per cent of their water.
But many countries do much better. Japan has a mandatory leak target of 10 per cent, which has been achieved in Tokyo through a major replumbing using stainless steel water mains and flexible joints capable of withstanding earthquakes. Singapore has reduced its leaks to 8 per cent with stainless steel mains.
As our table shows, there is little agreement about the best way to manage water: public, private and mixed systems are all in place, including, in France, public authorities sub-contracting to private companies.
Caution should be applied to some of the comparisons; the UK profit figure is the amount not retained within the companies; the Spanish salary is for the minister of public works; the UK salary does not include share options; prices are on slightly varied bases, and there are differing local supplements and penalties. For example, in Los Angeles, which has been used as the base for the Californian examples and which is recovering from a bad drought, consumers are allocated a set amount of water. If they exceed this, higher rates kick in. This being Los Angeles, the municipal water authority will pay for bottled water if supplies are cut off.
California offers another example of good water-conservation practice. During the recent drought, water companies cut use by fitting low-flush lavatory cisterns, sprinkler taps and low-flow shower heads free of charge. Thames Water thought about fitting free low-flush lavatories in 1980. It was cost-effective, but never adopted. Nobody in the company can now remember why not.