There are interludes in history around which history turns. Call them landmarks or forks in the road, call them tipping points, but their essential quality is this: they are not themselves the causes of change; the causes of change run more slowly and deeply than mere events; but something happens to bring things to a head. The right time or place, the right person – and bang, away we go. Such interludes deserve the attention of dramatists.

If so, the producers and cast of Consenting Adults have been lucky, for here is a human story too. Sex is the core crime and the police are intimately involved, and personality is at the heart of it too. And for all the fuss that has surrounded homosexual law reform since the 1980s – “Section 28”, the reduction of the age of consent to 18, then 16, and finally the Civil Partnerships Act – by far the biggest legal change happened 40 years ago, in 1967, when male homosexuality (which since the 19th century had been a crime in all circumstances, vigorously prosecuted by the police) was legalised for consenting adults over 21, in private. I was 18, and like most of my contemporaries, saw the new laws for what they were (and what the politicians kept insisting they weren’t): a fundamental change of direction.

But there was a difference between the way we saw things then, and the way a younger generation of gay men sees them now: a difference to which I shall return.

The 1967 Act would never have happened without Wolfenden. The name alone – shorn of “Sir John” – became a byword for the reforms. The establishment in 1954 of a commission to examine the law relating to prostitution and homosexuality, headed by John Wolfenden, and the subsequent report in 1957, 50 years ago today, pointed the way, and though the wheels of British politics ground slowly, the reform 17 years later (for which Margaret Thatcher, then very much a New Woman of her time, voted) owed almost everything to the report.

Owed everything, in fact, to a man who – played here consummately by Charles Dance – viewed homosexuality with the utmost distaste and never evinced the slightest enthusiasm for the case that, meticulously, laboriously and finally irrefutably, his own committee’s research established. This is the first of two vast apparent ironies that Consenting Adults explores. The second is that Wolfenden’s own brilliant, Oxford-undergraduate son Jeremy – played neither for titillation or tears, but with great honesty, by Sean Biggerstaff – turned out to be gay.

Jeremy was also an alcoholic. And he had a terrible, tortured yet fleetingly tender relationship with his cold but doting father. Who knows – nobody surely can – whether Sir John’s final, irritable resignation to the idea of reform owed anything to the mess he and his son were making of their relationship?

Jeremy was to drink himself to death, working for British Intelligence in Moscow. His father, knighted during his committee’s work, ended up in the House of Lords. Both have gone beyond recall. Neither was the type to write anything personal down. If Jeremy had not been gay, might his father have been less willing to accept the overwhelming evidence his committee encountered, that human sexuality and the criminal law simply didn’t mix? Or might he have been more relaxed in the face of the obvious?

Or did it make no difference at all? By all means watch Consenting Adults for the human drama alone. It carries you along. The script tries perhaps a little plonkingly at times to touch every contemporary contextual base, but, though bits have to be supposed or invented, I doubt that the film plays irresponsibly with the story; and the recreation not just of the look and sound of the early 1950s, but (important, this) the thrill of modernity hanging in the postwar air, is expertly conveyed.

At the end, though, ask yourself how free an agent Wolfenden really was.

I suggested that there exist catalytic interludes in history. This drama hints that the real catalyst for both the fact, and the drift, of the Wolfenden Report was not Sir John, but the British police. They were pushing things too far, arresting too often, entrapping too enthusiastically, and bringing to court too many of the British Establishment itself. Perversely, by enforcing the law too zealously, they forced a political class which would have preferred to turn a blind eye, to face the question of what the law should be. The answer was obvious: “uninvolved”. Wolfenden knew well enough (in that half-conscious way that Whitehall and its friends among the Great and the Good do know things) what he had been hired to do.

The “irony” of Sir John’s own personal reluctance to embrace the liberal argument was therefore no irony at all. The gritted teeth made his report easier to sell to doubters. Its subtext was: “We don’t like this any more than you do, but there’s no other way.”

The absolute minimum it was necessary to yield in the face of social and moral change was proposed in 1954 and accepted in 1967. The result was nearly 30 years more immobility. But the underlying social change went on. In the 1980s and 1990s I was lucky enough to be in politics and journalism at exactly the point (though my colleagues and the Tory and Labour whips at first insisted I was wrong) when something more had to give. And, just as happened with Wolfenden, it was finally the overzealousness of the police and of the reactionary part of the Conservative Party that forced the political world to review the evidence.

It was the outrageous behaviour of the Metropolitan Police in the early 1980s, prosecuting (through the use of entrapment by “pretty police”) under the law on “importuning” that goaded me into the campaign for reform when I was a young Conservative MP. When I was beaten up badly on Clapham Common one night I vowed to myself that I would never give up this cause. I wish now I had been brave enough to come out as gay, as Chris Smith later did. Chris’s decision was spur-of-the-moment: made when enraged by media attitudes.

And it was what came to be called “Section 28” that pushed people like Ian McKellen, Peter Mandelson, and many others, including me, into a positive campaign against persecution. Then, all at once it seemed, the log-jam broke – and look where we are now: almost embarrassed by our success.

By “we”, I mean my generation of gay men: the people who were teenagers when Wolfenden reported. I spoke earlier of a difference which sets me and my contemporaries apart; for, cold and condescending as the attitudes John Wolfenden embodied and expressed in 1954, the truth is that my gay generation were and remain closer to his mind-set than we sometimes like to acknowledge. Wolfenden and his work were all about making a concession: a concession to an unlucky section of the community, and a concession to the irrepressibility of the sexual urge. “We can’t help it – we were born like that” lay at the centre of our claim for tolerance, and it was a claim into which the Wolfenden Commission inquired endlessly, and largely accepted in the end. Gay men were supposed to feel, and did, gratitude for his acceptance that our natures were neither our “fault”, nor alterable.

Today a different thought lies at the heart of the campaign: “so what?” Intellectually my generation subscribes to that; but underneath we are still pulled a little Wolfenden’s way.

For the moral Right, the lesson of the 1980s echoes that of the 1950s: don’t push it. For liberals like me the lesson is equally clear: our best friends are not always the evangelisers but the realists: cold, calculating, compromising careerists such as John Wolfenden. He may have been a bloody awful father, but when it came to negotiating the twilight zone between Whitehall and Westminster, he knew just what to do.

Consenting Adults, BBC4, tomorrow, 9pm