Since the dawn of BBC TV news 50 years ago, successive prime ministers have sought to manipulate television to their own ends. Journalist Michael Cockerell, a veteran chronicler of Downing Street, charts the on-screen revolution and the pivotal role of a two-day-old calf.
When BBC TV News began half a century ago, Winston Churchill was prime minister. The Old Man called television a "tuppenny ha'penny Punch and Judy show," and never gave an interview, claiming it had no part to play in the coverage of politics.
To an extent, the BBC itself seemed to concur. It had no political editor, only a "parliamentary correspondent". The corporation's hierarchy thought the word "political" was itself too political, and it wanted to be above politics.
How things have changed. These days the prime minister and other leading politicians are all over news bulletins, and the BBC's offices in Millbank are a huge processing plant for political news.
But the past 50 years have been a painful learning process for both Number 10 and the broadcasters. Like pensioners puzzling over iPods, Churchill's successors struggled to deal with the new medium .
A polished act
"Coming into a television studio is like entering a 20th Century torture chamber," said Harold Macmillan, Tory prime minister from 1957-63, "but we old dogs have to learn new tricks".
Before Macmillan no-one had heard of a prime ministerial "image". He was the first to try to project one, with the help of prototype spin-doctors.
Macmillan had assumed the top job with trousers that disgraced his tailor, an unkempt moustache and teeth in disarray. Some 18 months later, as his official biographer put it, "a new self-confident Macmillan appeared on the screen. The disorderly moustache had been rigorously pruned, the smile is no longer apologetic and toothy and he is wearing a spruce new suit".
He was the first incumbent of Number 10 to emerge as a TV personality.
Yet "Supermac" was also first prime minister to discover the fragility of a TV image. His famed Edwardian unflappability came to appear fuddy-duddy and out of touch.
Opposition leader Harold Wilson made the most of it, presenting himself on the news bulletins as everything the failing prime minister was not. Wilson sought to project himself as dynamic and purposeful, having studied videotapes of the charismatic US president, John F Kennedy.
Wilson turned himself into the consummate professional. One TV newsman told me: "If a cameraman asked Harold to walk down the garden steps from the Cabinet room, across the lawn and stop on a certain leaf, facing in a certain direction, he would do it without sense of condescension - and get it right first time."
[Image maker Gordon Reece] was absolutely terrific - he said my hair and my clothes had to be changed and we would have to do something about my voice
But by 1976, Wilson had been party leader for 13 years and grown tired in the job. Likewise, viewers had grown tired of him. It's what advertisers and TV schedulers call "the wear-out factor". What is wanted is something new and different. And the woman who was to become the longest serving prime minister of the century had that in spades (or should that be pearls?).
Yet when Margaret Thatcher first became Tory leader, she reacted towards TV like a primitive tribesman faced with a white man's camera: as if somehow it would take her soul away.
So she took advice from Gordon Reece, a former TV producer.
"Gordon was absolutely terrific. He said my hair and my clothes had to be changed and we would have to do something about my voice. He was a real professional," she said later.
Reece introduced the election photo opportunity. Before 1979, no aspiring prime minister had given a news conference clutching a two-day-old calf in a meadow. Mrs Thatcher told me at the time: "The cameramen said they did not want a picture of me with a load of bullocks in superb condition. There was this beautiful calf. The media have their job to do and I am very conscious of that."
Fighting to stay on top
However, Tony Blair, with the help of Alastair Campbell and Peter Mandelson, took television to a different plane. When I gained access to Downing Street to make the documentary News From Number 10, I had the unusual experience of door-stepping the prime minister inside his home.
Tony Blair had come into his press secretary's office to talk to Campbell without realising we were filming there. When I asked Blair what he thought of TV news, he said the biggest problem from his point of view was to try and get a developed argument across to the public, "because I am lucky if I get a 30 second clip on the evening bulletins".
So, why were he and Campbell said to spend their whole time trying to spin the news? "It's just modern government now," Blair replied.
"There's a 24-hour-a-day news media. If a story comes out that says something and you haven't got the capacity to get on top of it and say: 'look, sorry the facts are x and y,' And, as you know, it's not as if these stories don't take a life of their own and then start running away into the far distance.
"And then the public thinks: 'oh, my goodness, why on earth are they doing that?' when you are not doing it at all. So you need to be able to get on top of the news, in so far as it's possible.
"But what matters to me most are to do the things that are really for the country... that's what I spend my time thinking about."
It was an impressive speech, although the effect was somewhat undermined by Campbell adding, on camera: "And that's why you have spent the past seven minutes talking to Michael Cockerell."
It was, I learned later, the moment the spin doctor wished he had bitten his tongue.
Michael Cockerell makes political documentaries for the BBC and is the author of Live from Number 10 - the Inside Story of Prime Ministers and TV.