Last Night's TV: Margaret Thatcher: The Long Walk to Finchley, BBC4

Sooner or later, every politician gets rehabilitated – or at any rate, the edge wears down on whatever ill will was held. Genghis Khan, maker of widows, despoiler of nations, can now be the subject of an admiring film that paints him as a sensitive and dedicated family man. One day, people will talk about Hitler with amused tolerance, and Stalin's main claim to fame will be as an icon for moustache wearers. Not that any of these cases is strictly relevant to Margaret Thatcher, who was, after all, democratically elected and is still admired by millions. But at the liberal-left end of things, she was loathed with extraordinary unanimity and ferocity, and it's interesting, that now, 18 years after she left office, she is becoming a darling of the media. The most recent issue of Vogue tried, in the face of common sense, to promote her as a "style icon" (mind you, the issue before had a piece about how Gordon Brown has been transformed from "dour, brooding Chancellor to genial PM", so this is clearly not an organ whose views on politics you're forced to take seriously). And now along strolls Margaret Thatcher: the Long Walk to Finchley, an account of her early career in politics by Tony Saint that manages to be both pro-Thatcher and anti-Tory, a trick previously regarded as too risky to be attempted by anyone outside a New Labour Cabinet.

Saint's strategy here was to treat Thatcher's actual political views as insignificant; what mattered was that she was not posh and, most importantly, a woman. This was brave Margaret's battle against the buffers, struggling from constituency to constituency in search of one that didn't want a war hero as its prospective parliamentary candidate, didn't assume that women are hysterical creatures whose place is, as one hostile old biddy put it, in the home, not the House. At several points in the drama, she was seen giving speeches to selection meetings, which contained windy versions of her views on trade unions and the Red menace. But it was clear that the constituency politicos weren't interested in these things – they responded with questions about local pothole black spots – and the film didn't really give us any chance to be, either. What we were supposed to be noticing was her conviction and sincerity, and the tutting and eyebrow-raising of old-fashioned buffers and matrons who couldn't look beyond her sex.

A case can be made for Thatcher as representing socially progressive forces. Michael Portillo made it after the film, in Dinner with Portillo, complaining that the Conservatives haven't been given the credit they deserve for electing a woman leader (perhaps that's why they've retreated now into their Old Etonian comfort zone). I'm not convinced. The essence of Thatcherism was a belief that success comes from individual effort, and surely she saw her own career as proof of that, rather than a blow for women in general. The Long Walk to Finchley papered over the cracks in its argument with a comic tone, implying that we weren't to take any of this seriously. Some of the jokes were neat. At a party function, the young Margaret, still only an aspiring MP, found herself dancing with Ted Heath, now safely elected – "You lead, I'll follow," she told him, for the only time in her life. At other times, anticipations of future events were contrived: young Mark Thatcher snatched his twin sister's copy of The Jungle Book – "When are you ever going to go to the jungle?" (If you don't get that, it's because you didn't follow Carol's success in I'm a Celebrity... Get Me Out of Here!, and you can award yourself a gold star.) After that, when a family day out at the seaside ended with Denis announcing that Mark had got himself lost among the dunes, my main emotion was weary relief that we'd got that one over with.

Even at its most jolly and watchable, it was hard to see what it had to do with the real Margaret Thatcher. The casting of the lovely, kittenish Andrea Riseborough was problematic; although she rose at times to a pretty convincing impression of Thatcher's hectoring tone and glassy gaze, she couldn't rid herself of a touch of irony – I never had any sense of the solid core of Methodist certainty. Elsewhere, the casting was erratic. While Samuel West managed a faintly uncanny reminiscence of Heath, both pompous and uncertain, Rory Kinnear's Denis was inexplicably lower class, with faintly strangulated vowels that sounded more like John Major than a successful post-war businessman who had come through prep and public school. This was one example among many of the drama's poor sense of its period, along with some cloth-eared dialogue: Denis asked Margaret, "How does that grab you?"; Margaret told constituency workers that there were "no no-go areas".

At odd times, I managed to suspend my disbelief long enough to start enjoying the story, but was always jerked back to consciousness by some clunking implausibility. As a serious enquiry into the most influential woman in the past century of British history, this was a non-starter. But as an ice-breaker, the start of a conversation about Thatcherism that doesn't degenerate into yah-boo-sucks, it was unmissable.

r.hanks@independent.co.uk