It seems so simple, logical even. At first glance. First, you just calculate how much a tax rate generates for the Treasury. Then you do the simple calculation to show how much more the Treasury would get if you put up the rate. Increase the rate by 50 per cent and – bingo – forecast that the receipts to go up by 50 per cent. Simples!

The real world is, of course, just that little bit more complicated. People, all of us, adapt our behaviour according to how much tax we pay. Watch passengers spend lavishly at the duty free shops in airports. Go to Calais and see the discount wine warehouses. Talk to your corner shop about how legal cigarette sales are falling (and