Punctuating this facetious drama about the late Princess Margaret was a bald actor playing the late Labour MP Willie Hamilton, who popped up from time to time to complain that her upkeep was costing the workers dear. We were periodically shown examples of these workers as they followed the royal soap opera at home and witnessed their feelings harden from loyalty, through puzzlement, to resentment. Margaret, louche, divorced and jobless, became, Hamilton told us, the poster girl for the republican movement.

Younger viewers of The Queen's Sister (27 November, 9pm) must have been puzzled, for the film displayed her as, on the contrary, the most immense value for public money, a priceless addition to the gaiety of our grim nation and the most entertaining turn to be offered up by the people who brought us the abdication until Diana Spencer bowled up in a see-through dress to ruin Charles's life.

If only Margaret's life had been public property in this way. But those of us who lived through her many scandals learned precious little of them. The press of the day, by modern standards, was supine and respectful, and for the most part the papers left her to it, which is why the gloating headlines spinning towards us on screen in this film were from imaginary red-tops and why the newsreels were mocked up. No, newsreaders did not, as here, read out public denials of her lesbianism on the 5.45pm news; royal correspondents did not appear on chat shows to discuss her divorce. For no clear reason, apart from narrative clarity, the writer, Craig Warner, simply imagined a culture of intrusion and disrespect that did not exist at the time.

His misreading was a pity, because the production designer, Michael Pickwoad, conjured up a colourful world of 1950s toffs, parties and nightclubs that looked highly convincing. The 1960s were summoned by Antony Armstrong-Jones's faux-prole photo studio, straight out of Blowup. And Roddy Llewellyn's Welsh farming commune, where Margaret occasionally went into hiding, was true to the 1970s in a Withnail and I sort of way. The acting was also credible. As the Earl of Snowdon (Armstrong-Jones was elevated in 1961), Toby Stephens caught the emerging sensibility of new 1960s man, bored by protocol, quite interested in his children and interpreting the permissive society as the go-ahead for taking birds on the side. He looked like the young Patrick McGoohan (if Granada still needs a lead for its remake of The Prisoner, he's their man). The always excellent David Threl-fall did not go for an impression of the Duke of Edinburgh, deciding to do Charles instead. His chilly bonhomie, thinly covering profound cynicism about every aspect of "the firm", defeated his sister-in-law's mood swings every time.

Keeping the drama watchable was a cut-out-and-keep performance by Lucy Cohu as Margaret. Aided, admittedly, by some good work from the make-up department - the skeins under her eyes grew each time she took her sunglasses off - Cohu aged Margaret from spirited girl about town to wizened harridan without ever breaking character continuity. Near the end she managed to invest a scene in which, the morning after a druggy night before, she commanded her shoe to "run off now" not merely with comedy but with pathos.

Warner's stern line was that Margaret's self-destructiveness was an aspect of her selfishness. In mitigation, it was suggested she had a bit of a father complex, the old king being the only one who recognised her as an individual. Being "locked up for seven years" in Windsor during the war "explained" why she was later so keen to take holidays from her background (as in the terrific scene where, putting on an Eliza Doolittle voice, she ordered a couple of ales in a public bar and then punched someone for slighting the House of Windsor). As the public's enchantment with her faded, she refused, it was repeatedly suggested, to accept that, as Snowdon put it, she was "B-list". Yet this craving for celebrity seems anachronistic, too. Margaret, unlike Diana, was of the generation of royalty keener that people curtsy to them than ask for their autographs. In a couple of scenes, Margaret even took to the cabaret floor. When Harold Pinter saw Shakespeare in Love, he remarked to Tom Stoppard he never knew Shakespeare was such a good runner. I never knew Margaret was such a good singer.

The production grew in depth and melancholy as it went on but was hampered by concluding at the outset that Margaret's soul was not worth saving. An early (probably faked) announcement from the Palace proclaimed that after the king's death the newly renamed Queen Mother would take up residence with Margaret, "who stays the same". And stay the same, according to this drama, she did. Even when she was strong-armed into giving up Peter Townsend, we were not allowed to believe her heart had been broken, or, indeed, that she had a heart to break.

I defended Alistair Beaton's savage satire on Blunkett, A Very Social Secretary, on this page a few weeks ago, but this posthumous damning of Princess Margaret seemed not only ungenerous but cowardly. That it did not have the guts to feature the Queen, who stays off camera and beyond criticism, reinforces the impression.

Andrew Billen is a staff writer for the Times