City bankers won't move to Zug – there's no Spearmint Rhino. Such arguments weakened with the 50p tax rate. The UK government is now toying with a windfall bonus tax. Coshing the bankers will be satisfying. Making people feel better about public service cuts is savvy. It will also help in places such as Teesside, where thousands may lose their jobs because of Corus's closure of the area's last steelworks: the government is vulnerable to charges it bailed out the banks but failed to do enough for the manufacturing firms that buy British steel.

But making any windfall tax fair and feasible will be tough. Widening the net to catch not only banks that took public cash, such as Royal Bank of Scotland and Lloyds, but also those that only benefited indirectly, such as HSBC and Barclays, as well as UK subsidiaries of, say, Goldman Sachs and JPMorgan, would damage the City. Expect rampant avoidance. Many will simply demand higher salaries. As for the non-doms, they will escape any levy altogether. It is easy for a global business to pay them outside the UK.

Without a co-ordinated move from the US, it would be madness for the government to lead the mob in stamping out the City's advantages as an entrep?t. The danger is of competitive populism. George Osborne, shadow chancellor, is refusing to rule out his own windfall tax and demanding that banks forgo their tax losses. Any windfall levy, finally, would be a distraction from the underlying problem: banks can pay vast bonuses because they enjoy economic rents as taxpayer-backed entities. Tackling any oligopolies, while increasing capital requirements and liquidity buffers, will have a more lasting effect than a one-off tax on the bankers themselves.