Dame Stella Rimington was the first woman to rise to the top as Director General of MI5. Her career, which spanned 25 important years and covered all three branches of the service (counter-espionage, counter-subversion and counter-terrorism), involved her in many of the key events and social changes of the second half of the 20th Century. Now a bestselling author, Rimington has published her autobiography and written a series of Liz Carlyle spy novels. Breaking Cover is the latest title in this successful series. Here she talks to Peter Guttridge, a fellow crime writer and former crime fiction critic for the Observer.