It is time for one final flourish of the magnifying glass: to detect the diabolical geniuses who were key to this criminal cornucopia of puzzles and enigmas. Huge thanks to Dr Clare Smith, manager of the Metropolitan Police Heritage Centre, and guardian of its archive, who expertly set me on the trail of many historical mysteries; also to the dazzlingly fiendish masterminds Roy and Sue Preston who devised and set the ingenious puzzles. Thanks to copyeditor Sophie Lazar, and proofreader Sally Sargeant, whose keen eyes could open a safe from a 100 yards, and of course, thanks to top publicist Jessica Farrugia and marketing guru Vicky Abbott, for getting the word out on to the street. Lastly, thanks to Katie Packer, with her formidable forensic skills; my agent Anna Power, who knows where all the bodies are buried; and Sarah Emsley, the ringleader, and brains of the entire operation.