Brenda Maddox does a good impersonation of Margaret Thatcher - the husky voice, the insistent tone, the complete confidence that she was right. About everything. Maddox met her twice - at a lunch at the Economist and at a reception at No 10 - and the PM blew through both gatherings like a hurricane. "I saw the phenomenon; it was quite terrifying," says Maddox.

She has now tried to capture the phenomenon in a biography that accompanies a four-part ITV series. Both book and series - based on interviews with politicians, friends and, most revealingly, childhood acquaintances - are about "Maggie" rather than Margaret Thatcher, remoulder of British politics. This is a search for the private person behind the handbag, hairdo and stridently rightwing opinions.

One problem with the search is that, as with many politicians but quintessentially with Thatcher, there is very little beyond the quest for power; she is not a politician with what Denis Healey called a "hinterland". "It is," as Maddox admits, "a monochrome personality. She was driven and obsessional, and had no life outside politics. She would come back early from holidays, and her only relaxation was wallpapering. She had to keep busy, and enjoyed doing up her daughter Carol's flat."

Thatcher appears to have been born terrifying. At junior school, she was cast as an angel in a nativity play and had to sing a solo rather than play the piano, at which she was already proficient. But she insisted on playing and singing. "Do you think she'll be a concert pianist," a friend asked her mother, Beatrice. "Oh no," Beatrice replied. "We've got - she's got - higher aspirations than that."

She has spent the rest of her life getting her way. "You are thwarting my ambition!" the 18-year-old Margaret Roberts told the headmistress of her school when she advised her to delay her Oxford entrance exam for a year to brush up her Latin. Once at Oxford, she experimented with wine and cigarettes, but quickly abandoned them, spending the money on the Times instead.

Maddox was not a fan of Thatcher before she embarked on the book, but appears to have warmed to her in the writing. "It fascinated me that anybody could be so split," she says. "There are so many anecdotes that attest to her niceness: she never forgot if your mother was sick and the following month would send you a letter in her own hand asking how she was. How could that coexist with her extraordinary harshness and rudeness, her sink-the-Belgrano unswervingness? She was very divided, yet completely unworried about the division."

The character Maddox really warmed to, however, was Denis Thatcher, whose earthy humour helped to temper the frostiness of Margaret Roberts. They married in 1951 and Denis supplied his young wife with a settled home life and the money to pursue her political ambitions. The strength of their marriage is the rock on which Thatcher's political career was founded. As soon as Denis arrives on the scene, the book's narrative finds a comic edge. When they move into Denis's flat following their honeymoon, Margaret sets about reorganising the kitchen and finds that the oven has no bottom. "I'm not surprised," says Denis. "I keep the gin in there."

"Denis is very funny," says Maddox. "It was a great temptation, especially when you got into deep things such as monetary policy, to bring in a one-liner of Denis to close the scene, because he was always there with it. Whether he actually made Margaret Thatcher laugh the way he makes everyone else laugh I don't know, but he certainly did bring fun into her life."

Maddox is good on Thatcher's 10-year slog to find a seat - it is easy to forget how suspicious selection committees were of young mothers who wanted to become MPs. She finally won through in Finchley in 1959 and alerted the London Evening News. A photograph of her standing in front of Big Ben was captioned: "Mark's Mummy is an MP now."

She took her maternal duties very seriously. "A friend of Carol's came home from boarding school with her for half-term," says Maddox, "and Margaret had worked out a programme for every minute of every day."

Maddox puts that control freakery down to Thatcher's own mother, Beatrice, who she believes had a bigger role than she is usually accorded. "It is easy to write women out of the picture, and Mrs Thatcher helped to do that with her own mother. But you only have to look at her to see her mother's influence - the homebuilding, the fussiness, the solicitousness, the meticulous appearance, not a hair out of place. She didn't learn that from Alderman Roberts."

Thatcher's earlier biographers, notably John Campbell and Hugo Young, hold sway on the operation of the 1922 committee and the ramifications of the Westland affair, but there is no doubt that Maddox wins on hair, clothes and cooking (Thatcher could produce a mean lobster flan). "She knew the importance of image from the very beginning and realised that women are judged by their bodies, their physical presence," says Maddox. "She had to put time into the way she dressed, but she got it right - she mastered power dressing before the phrase was even invented."

She is also good on Thatcher's flirtatiousness. "She used her femininity and did like younger men [such as John Major], but there's no sign that it got out of bounds. The younger, handsomer men she liked were no threat to her, and she wasn't so flirtatious with people of her own generation who might supplant her."

The aspects of the book that most interested the press ahead of publication were the vitamin injections in her bottom and her alleged all-night whisky-drinking sessions with her personal assistant, Cynthia Crawford - both designed to stop the Iron Lady from rusting at times of stress. Maddox, however, defends her subject against charges of excessive drinking.

"There are a lot of references to drinking, and people say she mixed a very good gin and tonic at lunch, but I never heard any accounts of her - or even Denis, who obviously did drink much more enthusiastically - being out of control or having to be put to bed. She did stay up late, she did drink whisky, but next morning she was always ready for work and impeccably turned out."

Maddox sees her fall as inevitable. "People were going to Denis and asking him to tell her it was time to go, but he said, 'I know you're right, but you tell her. I love her, I can't tell her.' Being pushed out was probably the only way she would ever have gone." She says many former members of Thatcher's cabinet are now "gratuitously rude" about her. "The consensus is that she saved the country but broke her party."

She likens the PM's final months and that tearful farewell to a Greek myth. "Somebody comes out of nothing, they rise, they stay on too long, they're surrounded by courtiers, they don't listen, they don't realise they're being undermined and then suddenly they're gone. That's the fun - or the sadness - of biography. You can see them approaching their fate and they can't."

· Maggie is published by Hodder & Stoughton (£20). The TV series is on ITV1 at 9pm on Thursdays.