In G.K. Chesterton's The Man Who Was Thursday, a secret policeman who infiltrates a violent seven-man anarchist gang in Edwardian London discovers that all the other members apart from Sunday, the mysterious ringleader, are secret agents too.
Scotland Yard would not need to go to so much trouble these days. It could simply, with the approval of the courts, listen to the anarchists' phone calls, scan their e-mails and place software on their computers to monitor their internet use.
The shift over the past two decades towards the use of e-mail, the internet and other technology – by both companies and individuals – has provided a potential trove of data for security services and governments. Were it not for barriers such as privacy laws, data encryption and companies' scruples, the world would a far easier place for police and spies.