The Tories had their jolly tearoom foghorn, Alan Clark. Labour has its more self-effacing, but equally telling, Boswell in Chris Mullin. His first volume of diaries, A View from the Foothills, which spanned the years 1999-2005, is one of my favourite Westminster memoirs, not least for the moment when, as a new minister, he excitedly receives a memo requiring him to attend a meeting, only to discover that it is marked "Low priority". At a time when we are awash with the memoirs of political grandees, the worm's-eye view is even more valuable.
This second volume covers the waning of New Labour. It's no less mordant or funny than the first - just a lot sadder. The 2005 government arrives reeling from Iraq and yet another bout of the TB-GBs. One of the great Blair-watchers, Mullin knows when he is witnessing a flawed master: "He gave a bravura performance. But we are in different territory now. Bravura performances aren't enough any more."
Having been dumped as an international development minister after less than five months in 2001, he is called in again by "the Man", who offers him an envoy's role in Africa. Junior jobs, notes a colleague in the same boat, "are sweeties to be handed out to keep the children happy". Ministers for Africa change at random. "We pretend to take Africa seriously, but we don't."
The random cruelties of politics are well documented here, a corrective to the view that MPs are all having a whale of a time at our expense. Most of those in the foothills are disposable chaff - lucky to be given a junior post and very likely to lose it in short order. Mullin is marvellous, too, on the insidious jargon of modern communication. New Labour officials talk of "the perception gap: ie, the fact that our constituents don't believe a word we say".
The best diarists coin asides that linger after the events they describe are long forgotten. There are plenty to choose from here. Glenda Jackson is spotted looking miserable and angry as always: "Goodness knows what she won an Oscar for. Certainly not her charm." Ted Heath is obituarised as "a grumpy old bugger". And as for today's secular saints, he dreams of strangling the "dreadful Joanna Lumley" for unleashing a tide of destitute Gurkhas with her good works. But he is also sneakily well connected - there are lots of conferences in bucolic settings and "lunch at Alnwick Castle" with the Percys - so not that averse to the toffs, as long as they have nice gardens in the north. There is also sincere affection for his Sunderland base. It's always a relief to find a politician who likes the place he serves.
Flaws? They lie mainly in Mullin's certainties, which are irritatingly unexamined. He doesn't like globalisers or free schools or outsourcing. After a few chapters of this, you wonder what life under a Mullin premiership would be like - a very polite Gosplan, perhaps. He never ponders why, if his socialism is so persuasive, more people don't want it. And he is a holy fool on the press, recommending ownership of "one Sunday [title] per proprietor, ownership confined to EU citizens only". This must be the most potent recipe for culling British newspapers ever devised.
Fortunately, he has a broader outlook on human dramas as Labour limps into the final stretch. He gets the advent of Gordon Brown just right: "He still doesn't look happy. What does it take to make Gordon look happy?" And there is no great lurking sense of optimism about the party's future. Tellingly, Ed Miliband, who now insists he was close to resigning over the government's decision on the third runway at Heathrow, berates Mullin for voting against it.
And so the "demoralised, depleted" Labour army stumbles towards the guns. Mullin picks away at the feeling that he has become "useless" and that the time in the foothills of power has amounted to little. These diaries resoundingly prove otherwise.
What a happy change they make from the boundless self-regard of those who reached the dizzying heights.
Decline and Fall: Diaries (2005-2010)
Profile Books, 416pp, £20
Anne McElvoy is political columnist of the London Evening Standard and a regular presenter of "Night Waves" on Radio 3