Introduction In support of a 1990 UK House of Lords private members’ Bill – to enable construction of a barrage across an estuary in a politically high-pro le part of northern England – a complex hydrodynamic model had been used to assess the distribution of pollution, should the barrage be built. In the run-up to the General Election of 1987, then Prime Minister Margaret atcher had stridden across the industrial wasteland alongside the River Tees, cast her arms open, inviting her television audience to take in the barren scene, and proclaimed ‘We shall regenerate this’. e barrage, oddly enough, was to be the spark of that regeneration. e model had consistently forecast that pollution in the future would be shi ed out to sea, far away from the inland barrage. While the opponents of the proposed barrage did not succeed in preventing passage of the Bill, they were granted the right to request a sensitivity analysis of the model. In the course of that analysis a substantial error was discovered. When corrected, pollution was forecast to accumulate against the barrage. e group responsible for the model and its forecasts was obliged to issue a formal public apology.