Rupert Murdoch has been on better terms with more prime ministers than anyone else alive. He had so much to offer by way of influence and contacts the world over – at least until the hacking scandal flooded his empire – and if a politician wanted to meet him in private he did not let his love of news get the better of discretion.

On 12 February 1981, Mr Murdoch was allowed to double the number of national newspapers he owned, by adding The Times and The Sunday Times to The Sun and the News of the World. It was normal practice for any bid for a national newspaper to be held up while the Monopolies Commission investigated, but in this case Margaret Thatcher's government overrode objections from Labour and waved it through. Mrs Thatcher reaped the political rewards for the remainder of her time in office.

It could have been embarrassing for the Prime Minister if there had been any suggestion she had privately colluded with Mr Murdoch to ease his bid – but that was specifically denied in The History of the Times: the Murdoch Years, written by a Times journalist, Graham Stewart, and published in 2005 by Mr Murdoch's company, HarperCollins.

There was it was asserted: "In 1981, Margaret Thatcher and Rupert Murdoch scarcely knew one another and had no communication whatsoever during the period in which The Times bid and referral was up for discussion."

Documents being released on Monday by the Margaret Thatcher Archive demonstrate that they did indeed meet.

In fact, Mr Murdoch secretly met Mrs Thatcher for lunch at Chequers, on Sunday, 4 January 1981, with the specific purpose of briefing her about The Times bids, at a time when other potential buyers were showing an interest and Times journalists were hoping to organise a staff buyout.

Mrs Thatcher's advisers were acutely aware that the meeting had to be secret. A formal record was kept, and submitted to Mrs Thatcher the next day with a note from her press secretary, Bernard Ingham, vouching that "in line with your wishes, the attached has not gone outside No 10."

The record notes that "the main purpose of Mr Murdoch's visit was to brief the Prime Minister on his bid for Times Newspapers." Mr Murdoch told her that it was "a firm bid" for all the titles, not just the potentially profitable Sunday Times. In a hint of the dispute that tore apart the East End of London five years later, when Mr Murdoch sacked 5,000 print and ancillary staff to facilitate the move to Wapping, he also told the Prime Minister that he was gambling on his ability "to crack a particularly tough nut" in the form of the highly organised print unions.

Mr Murdoch also speculated on who his rival bidders might be. They included the tycoon Robert Maxwell, who later bought the Daily Mirror, and Sir James Goldsmith, father of the current Tory MP Zac Goldsmith. The note records that Mrs Thatcher "thanked Mr Murdoch for keeping her posted" but "did no more than wish him well in his bid."

Mr Murdoch wrote to her on 15 January, to inform her that "The Times business is proceeding well", adding: "See you in New York on 28 February." By then, the deal had been clinched.

Mrs Thatcher needed the media mogul's support because she was so desperately unpopular. Her subsequent success has obscured the extent to which her government was peering over the abyss in 1981. Chris Collins, editor of the Margaret Thatcher Foundation website, describes the documents, which will be accessible at margaretthatcher.org as "the personal archive of a person under great stress."

They include the findings of a private poll by the Conservative Party which – for good reasons – they kept under wraps. Support for the Conservatives was below 20 per cent, with 68 per cent of those polled saying that they disapproved of what Mrs Thatcher was doing, while only 27 per cent approved.

This was creating huge strains within the government and the party. The newly released documents reveal the existence of a "Group of 25", who wrote a collective letter to the Chief Whip threatening to vote against any economic measures that might increase unemployment. One of its leaders was Stephen Dorrell, then the youngest Tory MP and now chairman of the Commons Health committee.

Mrs Thatcher relied heavily on the Chancellor, Sir Geoffrey Howe, but even, they could not get on personally. That was noted by her advisor, Alan Walters, whose private diary is among the documents just released.

On 6 January 1981, he recorded: "Saw MT at 10.00. All affable and unflappable. What should she do about Geoffrey. Who else could she promote. No one."

The papers reinforce that impression that Margaret Thatcher kept her cool despite the immense pressure she was under, fortified no doubt by letters from admiring members of the public, such as one elderly survivor from the Titanic, living in St Ives, who hoped that Mrs Thatcher would be a survivor too.

The files also contain her handwritten note to a young girl who had written to Mrs Thatcher in distress about her parents' divorce. "My own children had a happy time," the Prime Minister claimed, "and I should like you to have the same. Perhaps you would let me know if you ever come to London and I could arrange for someone to take you to see Parliament."

There is also the text of a long interview she gave to a French journalist during which she reflected on what makes people happy.

"Sometimes I think things come too easily to people now and when that happens they get dissatisfied from having had far more than we had, which gave us great satisfaction," she mused. "A small treat, a small thing, a small gift, a small surprise, something like a half day out or going to tea, a small unexpected gift gave us enormous pleasure and these days they have so much money and it doesn't bring them pleasure."

No British politician did more than Mrs Thatcher to help the rich get richer, and yet all along she privately believes that money does not make anyone happy – except Rupert Murdoch, presumably.

Swiped: All the President's doodled men

When leaders of the world's seven richest nations met in Ottawa in July 1981, President Ronald Reagan was new to the job, and nervous. Margaret Thatcher, sitting alongside him, noticed how he doodled to calm himself. When the summit ended, the President left his doodles on his desk, so she picked them up as a memento and squirrelled them away in the flat above Downing Street after writing a note on them to remind herself where she got them. They are published here for the first time.

Andy McSmith

Lady's not for laughing (not at herself, at least)

Lovers of satire were flocking 30 years ago to watch a stage play entitled Anyone for Denis, which took the mickey out of Margaret Thatcher and her husband. This did not affect the Prime Minister personally until the embarrassing day when she was invited to a special showing, the proceeds from which were to go to charity.

Mrs Thatcher did not want to go, but was warned that a refusal would be interpreted as proof that she had no sense of humour, so she reluctantly agreed. How she felt about it can be seen from a newly released letter, dated 29 May 1981, from Michael Dobbs, then a political adviser. The word "no" can be seen written in Mrs Thatcher's handwriting five times across the letter.

She was particularly determined not to accept a suggestion that she and Angela Thorne, who played her on stage, should be photographed in identical outfits.

Later she learnt that she was not the only person who had been dreading the evening. When it was over, she received a handwritten letter from Angela Thorne.

"Meeting you before the show made me feel so much more relaxed, especially as you had indicated that we would be going through it 'together'. It must have been two hours of agony for you," she wrote. Mrs Thatcher replied saying: "I thought we both got through rather well! And attending the performance got far more publicity than the other things I do."