What’s true of spending is true of tax. For a quarter of a century, politicians have competed with promises to lower income tax. I well remember sitting in a BBC studio when Gordon Brown ended his last Budget as Chancellor with a flourish, cutting 2p off the basic rate. How, I wondered on air, would he pay for that. It took a while to answer that question. Long enough for some of the poorest in the land to notice that they were hundreds of pounds worse off since he’d also scrapped the 10p rate of tax. Alistair Darling now notes drily that “if something looks too good to be true it probably is”. As the next Chancellor, Darling had to compensate the losers. The cost of doing so was a staggering £6 billion.