When I first started to think about writing this book, I approached the Bletchley Park Trust to seek their support. I am grateful to Simon Greenish, the Trust’s former CEO, for offering it unreservedly. Simon introduced me to Mark Baldwin, whom I would like to thank for putting me in touch with the Welchman family. Nick Welchman’s loft held a treasure trove of his father’s letters and documents, which he had faithfully kept safe since his father’s death in 1985. Sadly, Nick passed away before this book was finished. Gordon’s two daughters, Susanna and Rosamond, have also kindly shared memories of their father with me. The Welchman family has generously donated a considerable amount of material, including personal items and documents, to the Bletchley Park Trust. At the beginning of the project I sought the advice of my friend and colleague Frank Carter. I regard Frank as one of the world’s foremost experts on the mathematics and technologies used at BP during the war. Frank’s advice and knowledge has been invaluable to me and he has contributed technical material which can be found in Appendix 2. I am grateful to John Gallehawk who directed me to a number of relevant documents in the National Archives. I would also like to thank Michael Smith for his insightful views on a suitable structure for this book. Finally, I would like to thank my lovely daughters for their interest and encouragement.