Now, in between all those broadcasts — often from the Today programme at breakfast through to the Ten O’Clock News after dinner, he has written a book about the trade of journalism (My Trade: a Short History of British Journalism, Macmillan, £20).

Most books by journalists are written for other journalists. Marr’s is written for readers and viewers who wonder how their news is reported and edited, and nobody who cares for the reputation of the British press will disagree with his conclusions.

There are good anecdotes and insights, a lot of intelligence and a great deal of affection for our grubby trade in Marr’s book. But the conclusions he reaches make melancholy reading. We journalists have become caricatures and must do something about it, he says. We are often seen as strawberry nosed voyeurs, liars, drunks and cynics, and it is our own fault. We can’t blame evil tycoons or proprietors since their influence has grown less. Nor can we blame the politicians and regulators since we spend much of our time trying to keep both off our backs. There are serious problems in journalism that need to be talked about more openly.

The first is about trust. Journalism’s problem, according to Marr, is less direct lying than “slimy misrepresentation”. How many quotes by anonymous experts or sources are invented or at least “improved”? “Every day of the week I read stories about subjects I have followed and find myself uneasy or suspicious about off-the-record sources which just seem too neat or convenient,” Marr says.

Another part of the trust problem, he says, is our reluctance to correct stories. Correcting them ought to be a regular daily act. The reader or viewer ought to be assumed to know that what we say about a speech, a political stunt, a murder or a new strain of a virus, is tentative, not final, and will need correction. Instead, we have the habit of leaving loose ends. We don’t go back often enough and ask: were we right; what actually happened next? Equally serious problems concern tone and, above all, exaggeration. If there is a medical doubt about something, we cry plague. If there are questions about politicians’ motives, personal behaviour or honesty, we tend to treat them as the moral equivalent of a serial killer and turn to the facts later.

Marr also castigates “general emotionalism”, the trick of wringing the facts to get the biggest emotional impact. The idea of news has altered. It stopped being essentially information and became something designed to produce an emotional reaction, the more extreme the better. “Shock; eroticism; fear; laughter; anger . . . a paper which could produce a ripple of reactions from page to page could not fail.”

It could be said that journalism is reflecting a more emotionally open, modern Britain. But emotionalism isn’t working: newspaper sales have crashed over the past three decades.

The solution, according to Marr, is in the brains and hands and soul of the British journalist. The owners and practitioners of journalism will react if it becomes accepted in the trade that a loss of trust and an increasingly hackneyed emotionalism are losing readers.

It is when Marr reaches his peroration, an attack on the journalism of reporters sitting in front of screens in airless offices under the lash to be productive, that I cheer the loudest. As he says, office-bound journalists are vulnerable to the PR machines, the conmen, the special interest groups and, above all, errors that have been trapped in electronic or paper files. What journalism needs is decently paid and experienced journalists — and more of them — who are inquisitive, energetic and honest and can be trusted to use their judgment.

“Time and again, newspapers have been saved by the scrupulous health correspondent who refuses to take a scare at face value; or the science correspondent who manages to show shades of grey in a controversial and complicated story about genetic research . . .”

This is only a short précis of a magisterial book that should be required reading for those who command the heights of the British press.

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