Seven Up is back this week. Not the fizzy beverage, of course, but ITV’s ground-breaking strand, charting the life of 14 socially assorted British folk at seven-year intervals. It is directed by Michael Apted, as always, the Coronation Street and World in Action veteran now more famous for films such as Gorillas in the Mist and Enigma. By now his charges have reached 49 Up (ITV1, Thursday) and the group is down to 11, which is not a bad retention rate.

The new series coincides with the celebrations for ITV’s 50th birthday, so executives should be bracing themselves for another round of unflattering comparisons between past glories and the dismal state of their present output. The first round came when they ran Melvyn Bragg’s clip-show series, The Story of ITV, as a pre-birthday party, just as Celebrity Love Island was floundering and Celebrity Wrestling was dropped. Now, in the week of the actual anniversary, we get clips of the same programmes in ITV’s 50 Greatest Shows (ITV1, Saturday), with hagiography-McNuggets from a different set of talking heads.

It features Seven Up, of course, and it must be a relief to have at least one decent documentary strand other than the South Bank Show still running, even if we have to wait seven years between doses. By now we are so used to seeing so-called “ordinary people” that it is easy to forget quite what a breakthrough Seven Up was. Filming a cross-section of society and allowing them to talk freely about their lives was a real novelty in 1964.

I’ll Do Anything to get on TV, Channel 4’s recent history of ordinary people on the box, showed two brief clips of Seven Up without naming it, with a snide comment about ordinary folk “being questioned by an Oxbridge man with a mike”. Channel 4’s rewriting of history as a heroic journey towards Big Brother and Wife Swap completely failed to recognise the influence of the Up series or the hidden radicalism of Apted’s intentions. He did go to Cambridge, it is true, but as he now recalls, “the velvet glove of the early films hid a fierce indictment of the British class system.” Echoing a Jesuit formulation about the fate of adults being set by the age of seven, the first show, a World in Action special, was meant to demonstrate the way children were trapped by their background.

So the return visits to the Cockney jockey Tony, the high-minded public- school-educated Bruce and the rest have been testing Apted’s hypothesis. In the end it has proved only partially true. Some children have enjoyed limited social mobility as adults, others haven’t. Many viewers were touched by the personal crisis of Yorkshire farmer’s boy Neil, who lost his way only to be rediscovered in 42-Up as a Lib-Dem councillor in Hackney. The strand has been more interesting as a reflection of changing British attitudes and mores, especially regarding marriage and relationships.

But perhaps its greatest significance is that now, 42 years after the first programme, the series seems so unremarkable. After its appearance the “socially aware” documentary blossomed; flies began appearing on the walls of every home and workplace in the country, each armed with a camera. The docusoap was born, turning ordinary people’s lives into cheap’n’cheerful drama, and then reality TV upped the stakes by using them as the cast of ever-more extreme forms of pantomime.

Every seven years the Ups have had to insert themselves into TV landscapes ever-more crowded with “ordinary people” doing non- ordinary things. And for all its neutral intentions even this strand has distorted aspects of the subjects’ lives, if only by bringing them unlooked-for fame. The age of the innocent documentary has passed, if it ever existed. For long-term fans, at least 49 Up should be like a school reunion, a chance to catch up with old acquaintances and reflect on a world that refuses to turn out quite the way we hope or expect.

Editor's choice

• The 100 Best Films of the Decade
• 70 facts you didn't know about Marvel
• The 50 Biggest Movies Of 2010