One fine morning early this year I had tea with Stephen Greenhalgh, Boris’s pleasant if perspiring deputy mayor for policing, and discussed the two great crime mysteries of the 21st century.

First: the weird fall in crime. For the past 15 years, all manner of crimes have been on the wane, even violent ones: carjacking, vandalism, burglary, murder — but why? Mr Greenhalgh was admirably quick to credit the Met, but given that it’s a global phenomenon, stretching across Europe and America, that seems unlikely.