In the public sector generally, these economic relativities have long been ignored or fudged with various allowances and flexibilities. But pay differences between the high and low-cost regions are greater than ever. Private-sector wages in central London can be 50 per cent above those for the same job in the north-east, reflecting not only the higher cost of housing, travel and other goods and services but also differences in the quality of life. Teachers’ London allowance is hopelessly inadequate, both in the amount it pays and the areas it covers.

When higher education and teacher training was free and grant-assisted, graduates who wanted to put something back into education could afford to do so for relatively modest salaries. Now, when many have funded their own education and training, running up debts of pound;10,000 or more in the process, pecuniary considerations weigh more heavily. Little surprise, then, that many London teachers contemplate moving out of the capital - or out of teaching altogether.

The time has come to reconsider national pay scales because they suck good staff away from high-cost - and often high-challenge - areas. There are still many skilled and dedicated teachers in areas like London, but not nearly enough. Andrew Oswald, professor of economics at Warwick University, found that Office for Standards in Education inspections show a clear gradient, with the lowest quality of lessons in inner London, followed by outer London and the regions where the cost of living is progressively lower.

Teachers’ national pay scales are not just unfair and impractical. They are also a threat to the quality of education in some regions.