Andrea Riseborough as the young Margaret Thatcher

It is the mid Fifties, and she has just been rejected for the umpteenth time as the candidate for a safe Conservative seat by a panel of male chauvinists. The self-made grocer’s daughter from Grantham is inconsolable. “I am better than them,” she wails to her husband. “I’ve always believed that with application and merit, you can achieve everything. But now I see you’ll always end up strangled by the old school tie. Damn their Establishment, damn the lot of them.” Tears roll down her face.

This is a key scene from BBC4’s compelling new drama, Margaret Thatcher: the Long Walk to Finchley. Covering her decade-long battle against ingrained sexism as she struggled to find a winnable Tory seat between 1949 and 1959, the film humanises the former Prime Minister in the same way that Stephen Frears’s film The Queen humanised the monarch. This is all the more surprising when you consider that the film is produced by the BBC, an organisation perceived by many — Lord Tebbit, for one — as being institutionally liberal and anti-Thatcherite.

As she fights the forces of reaction, Margaret Thatcher, played with rare magnetism by Andrea Riseborough, a 26-year-old rising star who recently appeared in Mike Leigh’s Happy-Go-Lucky, comes across as a highly likable heroine. OK, so she’s not quite Lara Croft, but she is still a formidable feminist trailblazer. She is, as Denis (Rory Kinnear) remarks the first time he meets her, “a force of bloody nature”.

“I like to take the standard perception of a character and turn it on its head,” says the programme’s writer, Tony Saint. “This is an affectionate portrait of a commendably strong young woman who loved her husband and children, and I hope people who loathed Mrs Thatcher are really surprised by it.

“We had a cast and crew screening of the film. It was full of media types, who are not Mrs Thatcher’s natural supporters. At the end of the scene where she destroys Sir John Crowder (her predecessor as Tory MP for Finchley) the audience cheered. I thought, ‘Oh my God, everyone is applauding Margaret Thatcher!’”

The drama had a similar effect on Samuel West, who gives a highly entertaining turn as Mrs Thatcher’s perennial rival and predecessor as Tory leader, Ted Heath. “I’m a socialist and, like Ted Heath, no fan of Margaret Thatcher’s,” says the actor. “But I was fascinated by this script because for the first time it made it possible for me to admire her. This drama crystallises her struggle and unites her friends and foes. In the end, much to my surprise, I found myself rooting for her.”

Riseborough was also won over by this Mrs Thatcher. “I don’t share her politics, but I came away respecting her as a person,” she says. “She managed everything from cooking Denis’s breakfast of two poached eggs every morning to running the country on four hours’ sleep a night. To become Britain’s first female prime minister was amazing enough, but to do it in the face of this oppressive old boys’ club was just incredible. It took me such a short time to feel real fondness for her.

“I’ve seen that horrible ‘I’ve just smelt a fart’ look that comes across some people’s faces when they hear her name. You never want to mention her at a dinner party because you’ll lose friends. After a while I stopped telling people I was playing her because everyone wanted to talk to me about it. For the first 50 or 60 conversations, it’s interesting, but after that it begins to get boring. Mrs Thatcher bled into my life because she arouses such passionate feelings in people.”

The film has a keen sense of humour, something that, in office, the real Margaret Thatcher seemed sometimes to lack. Written with the benefit of hindsight, it is littered with mischievous gags about what was to come. At one point her young son Mark wants to emulate his father by travelling. “Can I go to Africa one day?” he pleads. “I wouldn’t cause any trouble.” As Saint himself puts it, his film “has the feel of an Ealing comedy”.

The film has one further surprise up its sleeve: this version of Margaret Thatcher is undeniably sexy. She wins over one crusty old Tory grandee with a coquettish, leg-crossing exhibition that Sharon Stone herself would have been proud of. She even has a highly flirtatious relationship with Ted Heath. At a Tory fund-raising dance, steam virtually comes out of his ears when he erroneously believes she is propositioning him.

But executive producer Robert Cooper warns us not to get the wrong end of the stick about this scene. “It’s a joke! It’s the most unlikely relationship in the whole world! We thought it was really funny to suggest that the longstanding tension between these figures should stem from Ted mistakenly thinking that she was making an advance on him. But come on, who’s going to take that seriously?” For West, the scene is emblematic of Saint’s relationship with reality. “Something can be true, whether it happened or not. We can’t know what actually happened behind the scenes — it’s much more thrilling to act something from a writer’s imagination than trying to slavishly reproduce reality. This film is framed by reality, not limited by it.”

So what would the lady herself think of Margaret Thatcher: the Long Walk to Finchley? “I hope that she takes the drama in the spirit in which it’s intended,” says Cooper. “It portrays what she went through very accurately and without mockery. And I think she’ll enjoy the jokes as much as anyone else. I hope Carol will as well. As for Mark, where is he now anyway?”

Saint is equally hopeful that Baroness Thatcher’s reaction to the drama will be positive. “At the heart of this film is a portrait of unstoppable drive and determination. It doesn’t dwell on political ideology; it deals with those character traits. And as someone fundamentally lazy, I find them irresistible. God, I wish I had half Mrs Thatcher’s energy!”