I began writing when I was still in the British Foreign Service, and it was then understood that even if you wrote about butterfly collecting, you used another name.

In the '60s - and right up to the present day - the identity of a member of the British Secret Services was and is, quite rightly, a state secret. To divulge it is a crime. The Services may choose to leak a name when it pleases them.

I wrote 'The Spy Who Came in from the Cold' at the age of 30 under intense, unshared personal stress and in extreme privacy. As an intelligence officer in the guise of a junior diplomat at the British Embassy in Bonn, I was a secret to my colleagues, and much of the time to myself.

Love is whatever you can still betray. Betrayal can only happen if you love.

Writing is like walking in a deserted street. Out of the dust in the street you make a mud pie.

The merit of 'The Spy Who Came in from the Cold,' then - or its offence, depending where you stood - was not that it was authentic, but that it was credible.