Last Night's TV - Our Drugs War, Channel 4; Britain's Park Story, BBC4

Drugs kill, and you should just say no. Suzanne Dyer knows this as well as anyone: her son, Chris, died last year from a seizure caused by the "legal high" GBL, after a milder form of the same compound, GHB, was banned. In Our Drugs War, the documentary-maker Angus Macqueen followed Suzanne to her son's grave, where she sat trailing her fingers along the base of the headstone commemorating his 24 years of life. "I say that I love him," she said, her voice nearly breaking. "I'll say to him sometimes that it'll be all right, and I know he's in a better place. And that we miss him. 'Cos we do miss him. So much. I'm sorry he's dead and I'm sorry he suffered so much."

It is hard to imagine anyone holding greater natural authority on the conduct of the war on drugs. Suzanne Dyer knows that her son would be alive if he had just said no; she also knows that as public policy, that's a pretty inadequate approach. As she put it: "If love could have kept him alive he'd have been alive."

Macqueen's navigation of what he calls Britain's secret geography brought him into contact with a string of compelling witnesses, from dealers who memorise undercover police officers' numberplates to teenagers who view their school drug-education programmes with jaundiced contempt. It was Dyer, though, who remained the locus of his film's fierce sense that the real casualties in this endless, eye-poppingly expensive battle are our own foot soldiers, the men and women we send into combat against so powerful an enemy with their arms tied behind their backs by poverty and criminalisation. Certainly, her quiet good sense, her retention of logic in the face of an overwhelming emotional tide, was a stunning rebuke to the more hysterical of those without first-hand knowledge who take the opposite view out of political expediency.

That's not to say that those who disagree were altogether unheard. But their views were noted without being attended to, and they were far from the film's expertly manipulated sympathetic centre. Dressed in middle-class duds that made him stick out like a sore thumb at an Edinburgh council block where at least 60 per cent of residents use drugs, Macqueen presented himself as a tourist, telling us that he used to believe that all drugs should be illegal, too, but that now his eyes have been opened. In truth, though, that was a rhetorical elision. As far back as 2005, Macqueen made a film exploring the disastrous impact of the war on drugs – specifically cocaine – in South America: he is not the ingénue he seems.

That sort of sleight of hand is part of the reason the film is so powerful to those who already agree – but whether it will actually change very many minds is another matter, for there is no serious engagement with the most vocal proponents of the status quo. Morally, it's hard to blame Macqueen for this. After all, there seems to be no question among those tasked with making our current policy work that it is basically a fantasy. The war on drugs cost us £1.5bn last year, we were told, a vast proportion of it spent on enforcement. Experts say that you would need to stop about 60 per cent of the drugs that are smuggled into the country to have a real impact on use; in reality, though, to take one example, we currently get about one per cent of the heroin destined for Scotland. No one who talked to Macqueen about it, law enforcement officials included, seriously thought we have the slightest prospect of making much of an advance on that figure. In this context, the law and its supporters seem not so much an ass as an irrelevance.

It's a quandary. When a film is so wise, so grounded in specifics, composed in such controlled anger, it's a pity to limit its persuasive powers by not dealing with the counterarguments; on the other hand, why should the Ann Widdecombes of this world be given another chance to shape a debate that they already have by the scruff of the neck? It's possible that parts two and three of this series, which focus on the international consequences of the trade, will deal more in ways to make things better that might actually be politically possible, but I doubt it. Like all the most powerful reporters, Macqueen is motivated more by identifying the problem than the solution. Perhaps the second bit just isn't his job. There is nothing wrong, after all, in thinking that politicians are more obliged to ask someone like Suzanne Dyer what they can do to help than she is obliged to press the answers on them if they are not prepared to listen.

Those depressed by Our Drugs War could find some comfort in Dan Cruickshank's account of a rather more successful branch of public policy in the syntactically similar Britain's Park Story, which took us through the history of our public spaces, paying particularly fascinating attention to the social good that their progenitors were determined to engineer. The idea – and one that might remind those Big Society Tories that their patrician predecessors at least acknowledged that you had to spend some money to change things – was that public mixing could have general social benefits. I don't spend nearly enough time in my local park, largely on account of being an indoorsy, anaemic sort with a predilection for computer games. If Cruickshank offered to accompany me and talk me through it with the same educative zeal as he devoted to this sweet, public-spirited documentary, though, I'd switch off the console like a shot.