It counts as ironic that a bastion of free speech has to depend on the release of state papers for its official history.

Coming from abroad you'd find it odd, meanwhile, that a broadcaster dependent on a poll tax imagines it is anything other than state institution. But the British Broadcasting Corporation is, and always was, the oddest of beasts.

Some of its veterans cling to strange beliefs. One is that the BBC succeeds best when it pleases no one, politicians least of all. What better demonstration of impartiality could there be? Here's John Humphrys of a morning on Radio 4's Today, skewering the political class left, right and centre, as impeccably rude as Disgusted, Tunbridge Wells, could wish. When there is no deference there can be no bias, or so the story goes.

Yet admirers of the Corporation maintain, simultaneously, that it "embodies the nation". That notion frayed last year with Scotland's referendum. Which nation? Whose nation? How much impartiality could this British institution bring to arguments that threatened its own survival? "Not much," replied many Yes voters. Yet the phrase is still trotted out. It is near-explicit in Jean Seaton's sub-title, 'The BBC and The Nation 1974-1987'.

How do you embody the nation in general, however defined, by pleasing no one in particular? Hit programmes and "appointment viewing" in a fragmented media age might be one answer. An impatient response, born in the period covered by Seaton's volume, was to ask what business the BBC had in making shows that could as well be made by commercial broadcasters. Yet if that was the conclusion drawn, what justified the licence fee?

When such questions are asked even now, another version of nationhood is dusted down. In this, the BBC is integral, apparently, to "how we see ourselves". It is to the BBC we turn in moments of national crisis, celebration, or mourning. It is the BBC we trust. And if the Corporation is trusted, say those admirers, there can't be much wrong with it.

One problem is that in those "national" moments there is nothing to distinguish the BBC from a state broadcaster. The chances of Humphrys sounding brusque on the occasion of a royal wedding are slight. It isn't done. Nor, in contrast to the period covered by Seaton, does the Corporation these days risk controversy in its handling of Britain's wars. The hammering it took in the aftermath of Iraq and the Hutton inquiry saw to that.

Seaton's title, 'Pinkoes and Traitors', has probably caused an intake of breath in certain quarters. It is, after all, a Tory slur that began with Dear Bill, an old Private Eye column that parodied correspondence between Thatcher's comically right-wing husband, Denis, and William "Bill" Deedes, former editor of the Daily Telegraph. Some Conservatives will therefore consider the title typically partisan, despite quotation marks.

Nevertheless, with the 21st-century BBC cautious, let's say, in its dealings with government, it is worth remembering old battles. This is how it was: on one side there were those who echoed splenetic Denis, who demanded that the BBC be shut down, sold off, or at least taught its place. In the opposing trenches were public service broadcasters fighting for survival and, sometimes, for an ideal.

Labour had done its share of fuming over the BBC. The secret plan had been to abolish the licence fee - Tony Benn was all for it - and render the broadcaster dependent on the state. Instead, the arrival of a Conservative government in 1979, led by a woman who rarely doubted that the BBC was stuffed with left-wingers, is the heart of the tale. As Seaton writes, "In the seventies and eighties... for the first time since it was founded, the very existence of the Corporation was in question".

Typically, the new Prime Minister's views were incoherent. Until she spoke to commercial broadcasters - who didn't fancy the idea one bit - Thatcher couldn't see why the BBC should be preserved from advertising. She was no believer in journalistic balance - a deceit, she thought - but never doubted the Corporation's bias.

Like her followers, Thatcher thought the BBC was overmanned, its employees unpatriotic, its output slanted, and its ideas of decency lamentable. Her Tories also wrestled with paradoxes. The BBC "was expected to restrict its output to the elevated, because entertainment was intrinsically commercial". No Thatcherite explained why voters should stump up for programmes they didn't watch.

Seaton could be described as a critical admirer of the BBC. She does not doubt its right to exist, or its value, but she is alert to errors and arrogance. With economy and style she covers the period in which Charles married Diana, EastEnders arrived, Bob Geldof demanded alms for Africa, Northern Ireland was torn apart, and Britain went to war in the South Atlantic. Somehow the events described feel almost contemporary and yet, simultaneously, of another age.

A chapter on David Attenborough, "public service animal", captures that sense better than most. With Life on Earth, this "true television intellectual" offered a series that Seaton considers revolutionary: fully 13 hours of BBC2 programming devoted to what was, in effect, an extended essay on Darwin.

True, by 1979 Kenneth Clark's Civilisation and Jacob Bronowski's Ascent of Man had established a model of TV seriousness, encouraged by Attenborough as channel controller. Life on Earth was different: it was serious and vastly, enduringly popular. It vindicated the BBC. No other broadcaster could have pulled it off. But there's the rub: would the Corporation contemplate such a project now?

The arrival of EastEnders in the mid-1980s seemed to provide one answer. You didn't have to be a Thatcherite to wonder why the BBC was entering the soap opera business. Perhaps the makers truly did know their audience, much as those assigned to the coverage of the Charles and Diana nuptials knew theirs. When the cameras caught a princess in tears during rehearsals, steps were taken to ensure that the footage did not reach BBC News. Today, journalists on the royal beat need no such lessons in deference.

These days the Corporation is controversial only in its own defence, whether over star salaries or the failure to deal with the aftermath of the Jimmy Savile scandal. In the 1970s and 1980s, faced with a government given to defining the nation in terms of the national interest, tests more fundamental were met. The BBC stood its ground when attempts were made to prevent journalists from describing Bloody Sunday in Derry as a massacre. It had to fight back when - obeying guidelines - Newsnight described troops in the Falklands as British rather than "ours".

Seaton concludes that Thatcher's animus towards the BBC had less to do with contempt than with a kind of perverse respect. She valued its place in public life; what she hated was its failure always to support her. The Corporation had an enviable authority and integrity, contends Seaton: "What mattered was its place in British lives".

Whether that can remain the case in a multi-channel world dominated by social media and a distrust of the old editorial hierarchies remains to be seen. Whether it should remain the case, for the sake of the BBC or its audience, is a question yet unanswered.

Pinkoes and Traitors - The BBC and The Nation 1974-1987 by Jean Seaton, is published by Profile Books, £30