It is said that a little knowledge is a dangerous thing. Lack of it can be more dangerous still. That is nowhere better demonstrated than in the debate about the merits or otherwise of remaining in the customs union.

The arguments for leaving appear to have considerable force. Within it, we are unable to make our own trade policy and suffer the effects of high tariffs; outside it, we will be able to forge our own agreements, reduce tariffs and benefit British consumers.

With average World Trade Organisation tariffs on clothing at about 10 per cent, on food around 20 per cent and much agricultural produce at higher levels still, the opportunities look substantial. The impression given at least implicitly, and often explicitly, is that