‘Are you moving out?” I ask Chris Mullin as we step over boxes to get into his small, chaotic office at the House of Commons. “Oh, no,” he says, surprised. Apparently it’s always like this. He clears files and papers from a chair for me to perch on. A card marking his 54th birthday is propped on a cluttered bookshelf. Another celebrates his 58th. “I like keeping birthday cards,” explains Mullin, who, at 61, is clearly not one of life’s spring-cleaners.

He is sufficiently together, however, to have published a giant volume of diaries. Rather alarmingly, it is the middle book in a trilogy that begins with the death of John Smith. “The publishers say they’ll see how this one does before publishing the earlier and later books,” says Mullin.

This one has gone down pretty well: it paints a gently critical view of the great and the good, with Cherie being bitchy, Gordon being inhuman and the Duke of Edinburgh being astoundingly rude. “Charles who?” quizzes the duke, when Mullin dares to refer to the Prince of Wales by his first name.

Throughout the book, people at Westminster really do seem to behave as if they are in the corridors of a minor public school: vignettes include Nicholas Soames slamming Tony Blair up against the lockers (for a laugh) and Peter Mandelson sending a whole range of people to Coventry (not for a laugh). “Marvellous place, very good-hu-moured,” twinkles Mullin, who, for all his far-left credentials (Tribune editor, member of the Socialist Campaign Group), went to a minor public school himself.

In entry after entry he characterises his Westminster work as utterly boring; when made a parliamentary undersecretary in the Department of the Environment, Transport and the Regions in 1999, he describes his job as “a mountain of tedium”, a “point-less activity” and a task “which has not the slightest influence over anything that matters”. Before long he is dubbing his new home the Department of Folding Deck-chairs. Compared with the inflated grandeur of the Alastair Campbell diaries, or the name-dropping of the celebrity-crammed Memories of the Blair Administration by Adam Boulton of Sky News, this approach is fresh, endearing and often very funny.

On reaching not-very-high office, he is handed an invitation for some event. “This is very low priority,” reads the attached Postit note. “I suggest we hand it to Chris Mullin.” In one diary entry he begs Blair not to forget him: “He promised not to, but we shall see.” Rising stars such as David Miliband are described with unstinting admiration: “It would be easy for lesser mortals to be resentful, were he not so obviously talented and infectiously likeable.”

Others get shorter shrift: John Hum-phrys is “an opportunist liberal who has never had to take responsibility for anything in his life”, Neil Kinnock is “much nicer now he is back in a job he can do” and even Blair (dubbed “the Man”) is subjected to a degree of mocking scrutiny; in one telling anecdote, Blair advises Miliband to “go around smiling at everyone and get other people to shoot them”.

Mullin sees no shame in being outside the new Labour core of spin and presentation; he delights in being slightly tattered and resolutely low-tech – “no car, no phone and no pager”. His is the view of the worm rather than the eagle.

“I heard a discussion on Radio 4 the other day in which someone remarked that failed politicians make the best diarists,” he says. Will he go down in the annals as a neo-Alan Clark? “I haven’t seduced three women from the same family, so I’m not in a position to compete on the same basis.” Does he feel a failure? “I’ve never ascended the heights,” is how he puts it. Still, there are benefits: “Mine’s a better place from which to look around. Would I like to have occupied a higher office? Yes. Am I in any way resentful about not doing so? No.”

He launches into an anecdote about how he might have been secretary of state for international development had he been carrying a pager. It’s the sort of Pooterish story that typifies Mullin, even though he seems to have been pretty sharp, writing three novels including the bestselling A Very British Coup and campaigning successfully for the release of the Birmingham Six.

Given this record, it’s rather surprising that his lasting legacy as a minister seems to have been provisions on the legal height of leylandii trees: “We succeeded eventually, but not in my time as minister. Eventually we got it added to the Antisocial Behaviour Bill.” He is still quite pleased about this triumph: “It appeared to command even the prime minister’s attention – albeit very briefly.”

In the diary, his younger daughter describes him as “a bit famous”. Does such a meagre ration of celebrity irk him? “Not really, no,” says the MP for Sunderland South amiably. “I’m quite happy with life. I’ve had a few little moments. If I’m remembered for anything it might be A Very British Coup or the Birmingham Six. And these diaries might stand the test of time.”

He has said he is not going to stand at the next election, even though his is one of the safest Labour seats in the country: “It was an on-balance decision and my wife’s was the casting vote.” Mullin and his Vietnam-born wife Ngoc have two children, Sarah, 20, and Emma, 13. “She has looked after the children and been a wonderful mother and I want to give her a chance to do what she wants,” he says.

For a long time Mullin aimed to coast into retirement by refurbishing a derelict 18th-century walled garden opposite Chillingham Castle in Northumberland. The chance to dig this garden was, for Mullin, “the glittering prize that eclipses all else”. Characteristically, perhaps, he failed to acquire it in 2005 when it came up for sale. Mullin writes: “That’s it, then. The dream is over. A life that might have been extraordinary will now be ordinary.” Why does he go on like this?

Maybe it’s because he was brought up to be modest and feels, in a very English manner, that it is unseemly to boast or name-drop (although there is a – blurry – picture of him and Nelson Mandela on the cluttered pinboard): “Well, I have a streak of pessimism in me. Let’s say I err on the side of pessimism.” About himself? “Both in relation to me and life in general. But I don’t on the whole care how I’ve ‘done’ in life. It’s not a very interesting subject and I tend on the whole to accept the cards life has dealt me. If you were very ambitious you might end up being disappointed.”

He cheers up when we get onto the subject of John Major. “I love John Major. He is a very decent, good man. And I think his career is truly, truly extraordinary,” he says, suddenly roaring with laughter. “He left school with no O-levels and did accountancy at night school and went to his first political meeting in Lambeth town hall, which was addressed by Sir Marcus somebody or other. Twenty years later,” Mullin hoots, “he [Major] made him a minister of state in his second government. Ha ha ha! I think that’s amazing! When he was defeated in 1997 his office was up on this corridor. He received a quarter of a million letters, many of them saying, ‘Nothing personal – we like you but couldn’t vote for your party.’ That said it all, didn’t it?”

As regards his own former leader, Mullin is stolidly loyal: “Piers Morgan says in his diary that he once wrote to Blair about ‘idiots like Mullin’ and how he’d had enough of people like me. And Blair responded, ‘Chris is one of Labour’s strongest campaigners and has unwavering tenacity in pursuing causes.’ I think that’s pretty good, don’t you?”

Well, maybe Blair realised he needed a good old-fashioned leftie like Mullin somewhere in his camp. “Oh, he didn’t need me,” says Mullin immediately. Honestly, it makes you want to slap him.

When I ask him why he thought the time was right to publish this highly entertaining portrait of British politics, he replies with classic understatement: “Practical considerations. We can expect four unexpurgated volumes of Alastair Campbell. Plus the Blair memoirs. Which I think will wipe the Mullin diaries off the face of the earth.” He pauses and allows himself to contemplate – for a tiny moment – whether there is indeed something special about Chris Mullin: “I am probably the only diarist from the lower foothills. Which is a unique viewpoint. Well, uniqueish.”

A View from the Foothills: The Diaries of Chris Mullin is published by Profile Books at £20. It can be ordered for £18 with free postage from The Sunday Times BooksFirst on 0845 271 2135

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