Growing up is hard to do, not least when you end up doing it watched by millions to see whether you’ve turned out to be a loser...

So, it’s refreshing to see some of the participants of 56 Up get a bit bolshy when they’re probed as part of an experiment they didn’t create.

In case you haven’t tuned in, 56 Up is the eighth film, being screened in three chunks, in the acclaimed Up documentary series.

Michael Apsted was as a researcher on the first film Seven Up in 1964 when he was in his early twenties, which asked 20 seven-year-olds from around the country their views on their own lives and futures. The inspiration was the Jesuit maxim ‘give me a boy at seven and I will show you the man’ and results in monochrome are captured for posterity (and the potential embarrassment of its subjects), such as cute suburban Liverpool lad Neil who declared he wanted “to grow up to be astronaut… or a bus driver”.

Apparently, it was Apsted's idea to clock in with the gang at seven-year intervals – though they were too young to sign contracts (their participation was voluntary and they were paid an undisclosed sum each time). This has resulted in one of the most pioneering reality TV experiments in history as the cameramen, and the godlike interviewing voice of Apsted, return for the latest slice.

As with many ambitious projects, there was an agenda. Apsted wanted to find out whether kids born into wealthier families have better prospects in life. And, while class is fascinating, in 56 Up, it is clear that trying to define people by where they’ve ended up on the social scale can be clumsy. For example, 21 Up showed public schoolboy John looking superior, indulging in everyone’s least favourite toff sport – foxhunting – but it’s (somewhat shamefully) not until this week’s episode of 56 Up that we find out he was orphaned at nine, brought up by a single working mother and won a bursary to study at Oxford.

The intimacy of some of the interviews Apsted has carried out, and the candour of the kids involved, has made many viewers feel they know them better than their own family. Several of the 7-Uppers on Monday said they were shocked at the public intrusion. Neil, the lad whose path via homelessness and mental illness to becoming a politician and a lay preacher in Cumbria has been documented this week, said that several people wrote to him after seeing his anxiety and depression on screen saying, “I know EXACTLY how you feel, when in fact, no one did".

Although Apsted’s intentions are clearly for the greater good and a better understanding of life in general, he, especially now, seems intent on picking the scabs of people’s woe if it makes illuminating telly. Some of his questions are hard to watch.

After suffering mental health problems since late adolescence, Neil at 56 is open about his suicidal thoughts in the past (“I don’t see any way out,” he says of his illness). But he’s coping, and has built a useful life for himself in spite of, or just possibly helped by, the humiliation.

“Are you scared of getting old?... … Are you happy?” The questions come clunking through the screen, but Neil never cracks or gets angry. While defending his privacy and remaining polite, reasonable and unedited, Neil’s answers make for very poignant viewing.

“Happy is a difficult word - how many people are happy?” he asks. “You’re probably only happy when happiness is the last thing you’re thinking about.”

The final instalment of 56 Up is on ITV1 on Tuesday (July 29) at 9pm, and all three are on iPlayer