When you begin to write a biography you feel like a young lover. If asked to go back to it long after it is finished, you feel like a dog returning to its own vomit. The relationship between biographer and their first subject is a weird one. In the beginning, you want to know everything. Years of research follow. No detail is too small. You interview friends, family and colleagues – until one day you realise you want out. You want to be able to go into bookshops and not look them up in the index of every book you pick up. Love turns to hate. The writing takes over from the research; it is the means of escape. The final manuscript lurches out of your printer. Proofs come and you are pulled back in. Then: it is finally over. You can no longer look at it, think about or change it. But part of you is already missing it. You can never be indifferent to the sound of their name overheard at a dinner party or spotted in a newspaper. But you no longer wake at 2am with the answer to the secret of their character revealed to you. They settle into the background of your life.

Years pass, and a publisher rings to ask if you want to do a second edition. Can you go back to your first love? This all happened to me and Hugh Gaitskell. In 1996 I wrote my first book on the former Labour leader. In August John Blake is bringing out the second edition. They asked me to write a new introduction. I procrastinated for weeks before I began. And then some of the old feeling surfaced. The infuriating paradox of the man came flooding back – all warmth in private, cold calculation in public. I thought again about his daughter crying as we sat in the National Gallery cafe doing an interview. Her bitterness as she said, “It is okay for you, you will move on to another subject; but this is our life.” It was the moment my publisher says I stopped pursuing stories about Hugh’s sex life. Actually, it was the moment I just wanted it to be finished. Eight years on I re-enter the world of old Labour and realise that the 1950s remain bizarrely real to me. Eventually I squeeze out 3,000 words. Now I am free again.

Not all biographers love or hate their subjects. There are 800-page tomes arguing that someone is a mediocre writer. Why not write a book about someone you think is a good writer or a terrible writer? My book taught me one important lesson. Write from hate. Write from love. But don’t write from indifference.