Investors are getting twitchy about the banks again. Several UK bank stocks, for instance, are at multi-year lows. But in a wider and more global sense, the twitchiness never really went away.

The gap between official policy rates and those at which banks lend to each other has been widening for several months. At the start of the year, it had subsided almost to normal levels. Now, although a little off the peak, it is still a good deal wider than when the crisis first struck.