The message from politicians has been simple. For Ken Livingstone it would be “miraculous” if “some terrorists didn’t get through” while for the Prime Minister and the Head of the Metropolitan police it’s a question of “when not if”. Their plea for us to be “alert but not alarmed” is difficult to internalise when the scenarios range from a lone suicide bomber, through a Madrid style attack, to an unconventional chemical, biological or nuclear attack. “Prepare for every eventuality” is a cliché that has ratcheted the nation into a febrile state. But won’t help you survive an attack. In amidst this maelstrom of fear, there is what Donald Rumsfeld might call “known knowns”.