Stand your ground, a former member of Margaret Thatcher's Cabinet advised me when I told him I was about to interview the then former Prime Minister.

"She will respect you if you do that."

"I have 40 minutes to find her soul," I said. "You'll make history if you do that," he replied.

I didn't make history but I did discover her Achilles' heel. In all the interviews I have done for newspapers over the years, I cannot recall one with a more revelatory moment. It happened when I was preparing to take my leave of her. While I was putting away my tape recorder I mentioned a book I had read on leadership. As soon as I told her what it said, she turned the tables and began asking me questions. She wanted to know more. What was this book? Where could she get hold of it?

The more she showed interest, the more it became obvious she was still bewildered by her expulsion from Downing Street three years earlier. And, despite the passage of time, the wound it had inflicted on her was hurting. Not such an Iron Lady after all.

Our meeting at the Balmoral Hotel in Edinburgh got off to a cold start. Her handshake was cursory. She was unsmiling. The photographer posed her at the piano and asked her to crouch. "This is a strange position," she muttered, frozen as if in mid-curtsey.

"Yes," I said, "the first man to whom you have bent your knee."

Her stare was cold. I knew what President Francois Mitterrand of France meant when he said Mrs Thatcher had Marilyn Monroe's lips but the eyes of Caligula.

As we sat down she realised the seat of her chair was too deep to allow her to support her back and rest her feet on the floor. "Why do people make chairs like this?" she asked. "For people like me with long legs," I answered. And so we settled – or squared up – for our chat.

After 20 minutes I had managed to ask only two questions. We were halfway through our time and she was doing what she did best: dominating the interview. I had seen her do it often with television interviewers. I wasn't faring any better. But as she talked about her childhood I realised people who defined her as having pulled herself up by her boot straps had missed the point.

Her father left school at 13 but during her childhood he was a person of stature in Grantham. He was a magistrate and mayor for a time. He was governor of several schools and was, according to Lady Thatcher: "The sort of man called upon to make impromptu speeches at the Rotary. Alf always had some thought to contribute."

His daughter bathed in reflected glory. She hero-worshipped him but what a stickler. If her classmates were going out and she asked to join them: "My father would say: never do something just because other people are doing it. Never adopt their opinions just to go with the crowd. Make up your own mind what you want to do and then try to persuade others to your view."

She learned that lesson. As she spoke I realised she was a corner shop princess. She was never one of the people. She was a cut above the neighbours – separated from her schoolmates, elevated above her mother and sister. Each week she and Alf went to the library to select biographies for the two of them – and a novel for her mother.

Her father, she said, told her that popularity didn't matter. What mattered was that people respected you. I said my own father would have agreed and she replied, "and although we are from different backgrounds".

There it was, that class consciousness she'd carried right through her premiership. In her memoir, The Downing Street Years, she attributed her downfall to the grandees of the Tory Party. Clearly she thought I had sprung from them too. This time I managed to interject that Douglas Home was my married name and my background was more like hers. Instantly she was friendlier. She was now engaged and co-operative.

I asked how she had emerged from a background that was echoed millions of time across the country to be talked about in the same breath as Elizabeth I of England? What made Margaret Thatcher different?

"Passion," she said. So that was it. A brew of passion, her alderman father's dictum of self-reliance and monetarism – and Thatcherism was born.

Why did Thatcherism fail in Scotland? I asked. "Thatcherism in its economic sense took off here," she insisted. "Look at the city of Glasgow. Look at the transformation. It did not take off in the political sense. When we came into office 50% of families lived in state or local authority-owned houses. It was a much bigger turnaround than elsewhere."

My time was up. My 40 minutes had over-run and I was no closer to her soul. Then I mentioned having read a sociology book which talked of the 13 characteristics of leadership. I said I thought of her when I saw the final characteristic was to be the scapegoat.

"Was to what?" She was suddenly at full alert.

"And what was the book? Really? And where did you read this?"

She asked me to give the details to her press secretary. It was three years since she had been ousted but she wanted the book.

I had hit a nerve. The people had never rejected her, she said. She wasn't voted out by the public. Clearly she was mystified by what had happened to her after winning three elections – and wounded.

By mentioning that sacrifice was an aspect of leadership had I provided her with a possible answer to her party's rejection of her?

I would be lying if I said I liked Margaret Thatcher by the time I left her. But Alderman Roberts would have been pleased for, though I didn't agree with her, I certainly respected her.

My final question was to ask if she had compassion for those unable to rise to opportunity. She said: "There are always some we have got to pick up and if you don't do it through education and through the voluntary work of the community ... we won't retrieve the next generation."

Through the voluntary work of the community, that sounds suspiciously like the Big Society. Margaret Thatcher died yesterday. But it seems her legacy lives on.