Ted Heath looked at me darkly. 'You've done a terrible thing, you know that?' I shuffled from foot to foot. 'Have I?' I asked. 'Yes,' Ted replied. 'You've made me feel sorry for Margaret Thatcher.'

Of course, this wasn't the real Heath - he died in 2005 - it was actor Sam West who portrays him in a new TV drama I have written about our first female Prime Minister.

A former member of the Socialist Alliance, Sam was never a fan of Thatcher, but the story that my script tells - her titanic struggle, against all the odds, to win a seat in Parliament in the Fifties - almost brought a tear to his eye.

To be honest, he wasn't the first to gently admonish me for changing their perception of the Iron Lady. In fact, I was getting used to this sort of thing.

Fresh face: Andrea Riseborough as Margaret in The Long Walk To Finchley

For what I had come to realise is that, in telling the story of so colossal a public figure as Thatcher, someone who changed the face of the country and who divides opinion so fiercely, the first thing you're up against is the mass of rock-solid prejudices that virtually everyone in the country holds about her.

So when the idea of a film was first proposed to me, I had to examine my own feelings about her. They were ambivalent.

I grew up in the Labour heartland of Newcastle, but my parents ran a small business and so were natural supporters of Thatcher when she came to power promising to help people just like them.

I was ten when she was swept into Downing Street in 1979 and even at that age, with the lights darkening and the TV suddenly going off in the middle of Blue Peter, I was vaguely aware that maybe something ought to be done about those naughty unions.

But then I lived through the education cutbacks of the Eighties and began to feel that some of her other policies were starting to be driven by a personal agenda rather than the good of the nation.

Thatcher meets a voter in a pub in Finchley in 1959, just before she became an MP

By the same token, I could never fully understand the level of vitriol she received from some quarters.

I can remember how surprised I was by my own sense of outrage at seeing her represented on Spitting Image dressed in a Nazi uniform.

I never subscribed to either extreme view of her, which left me feeling oddly well placed to approach her as a character.

What also appealed was the chance to write a story about Thatcher that ended in 1959, when she fulfilled her dream and was elected MP for Finchley.

Everything that was to happen subsequently in her story became an accident of history as far as this film is concerned.

This was the chance to write about her without the baggage of ideology or the shadow of events such as the Falklands War or the miners' strike.

This hugely important decade in her life presented the perfect opportunity to tell a story that would be neither hatchet job nor hagiography, but a representation of a real young woman - independent and strong-willed but feminine and vulnerable at the same time.

Andrea gives a complex and sympathetic portrayal of the young Mrs Thatcher

But what I also realised was how little I knew about this early part of her life.

Yes, I was aware that she had married Denis, that she'd given birth to twins not long afterwards and that she had given up a career as a scientist (who, incidentally, had been involved in the invention of Mr Softee ice cream) to study and qualify for the Bar.

But as I began to read books about her life, I didn't feel a great deal wiser.

There was a strong sense of omission, with biographers perhaps keen to get on to the juicier material of her time in Parliament, then government.

Even studies among the Conservative Party archive gave only glimpses of a young, ambitious woman fighting to fulfil her dream of reaching Westminster.

But what glimpses they were, like the letter she wrote to Tory Central Office asking for her name to be withdrawn from the candidates' list following her failure to win selection in her local town of Orpington, Kent.

She wrote: 'The only political temptation for the next ten years was Orpington for which I have long had an affinity. Now that temptation has been removed for all time, I shall continue at the Bar with no further thought of a parliamentary career for many years.'
It's a surprise you can't see tearstains blotching the notepaper.

Or the revelation that as candidate for Dartford, she had fought the 1950 Election in the neighbouring seat to a certain war hero and heavily touted political prospect - Edward Heath.

I was amazed to learn that two people, who had each gone on to become Prime Minister and whose political fortunes were destined to clash dramatically when
Thatcher took the Conservative Party leadership from Heath in 1975, had known each other as far back as the late Forties. It threw a whole new light on their famous spat.

But there were two things that came through crystal-clear in all my research.

First of these was the force of character that the young lady from Grantham displayed from the outset.

Parachuted in to contest the hopeless seat of Dartford, she gave it absolutely everything and managed to cut the huge Labour majority by a third.

Even at such an early age, she displayed a flair and energy for campaigning that was second to none. In June 1949, for example, the Daily Mail caught up with her pulling pints in a working men's club, as it was the only way a woman was allowed in to such establishments.

Double act: Andrea with Rory Kinnear as Denis Thatcher

Margaret was always ready to go where other Tories feared to tread.

She moved on from Dartford with great expectations of finding that all-important winnable seat.

But while her determination, belief in her own abilities and sheer enthusiasm impressed plenty, it sent just as many Tories running for cover.

Time and again her talents were ignored and she faced awkward questions about how she would balance work and family life - questions the men she competed against never had to answer.

In one constituency she was rejected for selection because she was too 'intense'. This confirmed what I already knew - that I had a great central character to work with.

The other factor was the size of the immovable obstacle in Margaret's path to Parliament, an immovable obstacle otherwise known as the Conservative Party.

Canterbury, Orpington, Beckenham, Hemel Hempstead, Maidstone - the list of vacant seats that turned her down is a roll-call of her growing frustration as she came face-to-face with the male, public-school Establishment that epitomised the Tories of the post-war period.

It's interesting to note that in those constituencies where she failed to be selected, the applicants who beat her all went on to be notable for their spectacular lack of parliamentary achievement.

Antipathy: Geoffrey Palmer plays Sir John Crowder, Thatcher's predecessor in Finchley

They were all happy to rub along anonymously on the backbenches.

The reality at that time was that it was the right school tie, a certain clubbable quality and a 'good war' that counted enough for constituencies to overlook Thatcher's political abilities and relentless enthusiasm. That and being a woman.

As I learnt more about the prejudice and opposition that she had to overcome, I felt her frustration and began to admire her stoicism in refusing to succumb to it.

And the story was populated by characters from a time unburdened by political correctness.

Some of their reported attitudes to her are hard to believe in these more progressive times, although I have to admit that they made those characters a lot of fun to write.

Take Sir Waldron Smithers, the bibulous MP for Orpington until his death in 1954. One anecdote relates that when the date for an important debate was announced in the House, Sir Waldron, having lunched particularly well, rose to his feet and declared: 'Thash my birthday!'

His response to Margaret Roberts, as she was then, becoming the party candidate in Dartford was less cheery.

'Couldn't they have got a local businessman to stand?' he apparently wondered out loud, even though he knew the seat was a dead loss.

Sir John Crowder, the MP whom Thatcher succeeded in Finchley, also took a grave dislike to her and reportedly moaned that Central Office was giving Finchley a choice between 'a bloody Jew and a bloody woman'.

On the other hand, there was the more sympathetic and chivalrous Sir Donald Kaberry, Tory chairman of candidates, who supported Margaret and even offered her avuncular advice on what outfit to wear at a key meeting.

As I read more and more about the preposterous Tory grandees Margaret had to deal with in her odyssey, the clearer the idea became of the film I wanted to write.

Here was the story of a young ingenue in post-war Britain, trying to make her way but constantly frustrated by Establishment duffers who relied on their contacts and privilege to further their careers.

This wasn't going to be some bone-dry, painstakingly accurate docudrama full of long meaningful looks, narrated by a 'resting' actor with a solemn baritone.

By creating a tone that was playful and self-evidently not to be taken too seriously, I felt I could employ a certain dramatic licence, making a few speculations intended to amuse and entertain, not to be taken as gospel - especially in imagining how Margaret's relationship with Ted Heath might have played out way back in her Dartford days.

What I wanted to write would be a sprightly comedy-drama in the best English tradition, the sort of thing that Ealing Studios were lighting up British screens with at much the same time as Margaret, who was probably far too busy to go to the cinema, was working furiously towards her lifelong goal of a seat on the green benches of the Commons.

But the driven, determined Margaret is only part of the picture - these were also the years when she fell in love with Denis and raised a family.

I knew that, to take an audience with me, I had to create an attractive character to watch, one who had a life outside politics - even if her thoughts were never really far from the evils of surtax and Britain's loss of influence in the Nile Basin.

With the script finished, it seemed important that we cast a lesser-known actress as we were presenting a fresh take on such a well known personality.

Like me, Andrea Riseborough, who plays Margaret in the film, is from Newcastle and she has gone on record as admitting she was no fan of Lady T before picking up the script.

But her experience in investigating the truth behind Margaret's 'Long Walk' was much the same as mine and now she freely confesses to being an admirer of her character, if not her policies.

In portraying Margaret at turns as pre-feminist firebrand, domestic superwoman and blonde bombshell, she gives an astonishing performance that needs to be seen to be believed.

And so, with the film shot and edited, the acid test came when a special screening was held for cast and crew - the first time anyone outside a handful of people had seen it.

I sat nervously at the back and waited. Would they share the sense of admiration for Margaret that I'd developed in researching and writing the film?

We came to a vital scene where Margaret, older and a little wiser by now, turns all her pent-up anger on the departing Sir John Crowder. In her ruthless destruction of him, we begin to see the first embryonic signs of the Iron Lady being forged.

Building up to a tumultuous crescendo, she leaves Sir John with a devastating verbal sideswipe, telling him: 'What your 25 years in the House amounts to, if I may say so, is not a parliamentary career but an impersonation, a caricature of one.'

As she turned and walked away defiantly, I sat forward, holding my breath and watching the invited audience.

Then it happened. Suddenly as one, the assembled group of actors, production staff and BBC employees started clapping and cheering. Now there's a thing, I thought to myself.

For what I realised was that, when it boiled down to it, they weren't cheering me or the film, or the script. They were cheering for her.

• Margaret Thatcher: The Long Walk To Finchley will be screened on BBC4 on Thursday at 9pm.