‘I always say, keep a diary and someday it’ll keep you.’ No one knows who came up with that line first. It might have been Lillie Langtry. It could have been Margot Asquith. What we do know is that the line was made famous by Mae West, who gave it to her character Peaches O’Day in the script for her 1937 film Every Day’s a Holiday.

Every day is a diary day for me and has been since 1959, the year I turned eleven and my great-aunt Edith (a Lancashire infant school headmistress) gave me a shortened (and thoroughly expurgated) edition of the diaries of Samuel Pepys. Inspired by Pepys’s example, I have been keeping a daily account of my life (and the passing scene) ever since.

It’s how I bonded with Tony Benn, veteran Labour MP, sometime cabinet minister and probably the most prolific diarist of his day. Benn befriended me when I was an Oxford student in the 1960s. He told me then that he had started his diary at nine: ‘Got up, had breakfast, it rained today.’ He encouraged me to keep at it and include as much everyday detail as possible. ‘It’s a dreadful burden,’ he said, ‘but you’ll be grateful in the end.’ He told me his diary-keeping was a bit patchy in the early years and that during the war it was illegal: ‘You weren’t allowed to keep a diary in case it fell into the hands of the enemy.’ 

I got to know Tony properly later, in the 1970s and 1980s, when we were near-neighbours in Notting Hill. I would go to have mugs of tea with him in the almost comically cluttered basement of his house, where there were several cramped rooms piled high with diaries, boxes of papers and unravelling spools of audio tape. ‘We’re living history,’ he’d say, beaming ‘and recording it as it happens.’ 

Tony dictated his diary in later years and acknowledged that this led to misunderstandings in transcription. He told me how he had found ‘cuddly Pooh’ in the middle of one entry and that it had taken him a while to work out that the sentence should have read ‘Hugh Cudlipp who…’ ‘And for one awful moment’, he told me, ‘I thought I’d had a forgotten relationship with a Russian actress, Zinovia Flotta, until I realised the words I’d dictated were “Zinoviev Letter”.’

Benn published several volumes of his diaries. When I was an MP in the 1990s, he encouraged me to publish mine, not least because I was a government whip and no government whip before me had ever published a detailed account of the workings of the whips’ office. ‘That’s the very reason you should get them out there,’ he said enthusiastically. ‘Publish and be damned.’ After leaving Parliament in 1997, I followed his advice.

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My Westminster diaries, Breaking the Code, have just been republished. They are not about policy. They are an account of the reality of an MP’s life, written either in the constituency, early in the morning in bed (with tea and Marmite on toast), or in the House of Commons, on the go, in the library, in the chamber or in committee. The rules of the House do not allow you to read books in committee, but you can write them. (If you are crafty you can, in fact, read them. A colleague completed War and Peace while serving on the Finance Bill Committee. He photocopied fifty pages at the start of each day in the library and brought them to the committee room tucked inside the Budget Red Book.)

When he was in government, Benn scribbled notes during cabinet meetings. Immediately on returning to his department he wrote them up, if necessary sacrificing lunch to do so. I followed Benn’s example. Occasionally, it proved a risky enterprise. Towards the end of the 1992–7 parliament, when the Major government was in freefall, I was witness to a late-night encounter between the Chancellor of the Exchequer and the foreign secretary, apparently at irreconcilable odds over a central issue of policy. In the Commons library, at 2am, mildly squiffy, I wrote my colourful account of the meeting. The following morning, sober and aghast, I realised I couldn’t find my papers (I don’t keep my diary in a book, but on lined sheets of A4). I called my wife. She searched high and low. I hadn’t taken them home. I called the government car service: my driver hadn’t seen them. Suddenly, I recalled where they were. Walking as rapidly as the whips’ rules allow (a whip never runs; everything is under control, always), I made my way to the quiet room at the end of the library. Standing by the table we sometimes shared, apparently leafing through my papers, was Peter Mandelson, Labour MP, already known as the Prince of Darkness. My heart stood still. Peter looked up and smiled. ‘I’ve found it.’ He had been looking for his Filofax. 

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A diary needs to be immediate and indiscreet. I was a friend of the actor Kenneth Williams and feature in his diaries – getting off quite lightly, I am happy to say. Kenneth could be waspish and often wrote things in his diary that he felt in the moment but might have thought better of in retrospect. 

Thinking of the political defections of recent weeks, I looked back over my diaries to see whether I had said vicious things about the defectors of my day. When Emma Nicholson defected to the Liberal Democrats in 1995, I wrote that she was ‘self-serving, self-regarding, and someone who regularly misses the point not only because she’s hard of hearing but also because she’s not as bright as she thinks she is’. I met her again the other day and thought she was quite delightful.

Along with Pepys, my favourite diarists include Chips Channon (Simon Heffer’s recent three-volume edition is a masterpiece), Noël Coward and Virginia Woolf (A Moment’s Liberty: The Shorter Diary of Virginia Woolf, edited by Anne Olivier Bell, is possibly the best book there is for the bedside table). A good diary needs to be written by a good writer, with an eye for detail. Siegfried Sassoon on 13 January 1921: ‘Rainy weather. Does the weather matter in a journal? Lunched alone; does that matter? (Grilled turbot and apple-pudding, if you want full details.)’ We want full details. Every time.