When George Jesse Turner, a cameraman in his 20s, was invited to join the team making a unique series of documentaries following the lives of a group of schoolchildren, he grabbed the chance to work with a young director who was making a name for himself at Granada TV.

But, by taking the job alongside Michael Apted 42 years ago, Turner was harnessing himself to a roller coaster that has played an exhilarating role in the rest of his life. This week, the series that started out in 1964 as Seven Up! comes back to our screens for a ninth instalment, and Turner, 74, who has filmed all but the first two series, is calling for a suitably grand finish to the groundbreaking run of documentaries.

“The proper way to end it might be a fantastic party. But that could cost up to £50,000. So it will be up to ITV,” he said.

The 63 Up series, which starts on 4 June, tells the life stories of 14 people plucked from varied backgrounds in the 1960s for what was intended as a one-off documentary. Seven years after the first programme, Granada TV suggested that Apted go back to interview the subjects again – and so the framework for one of the world’s most influential documentary experiments was established.

“I have my passions,” said Turner. “I have my grandchildren, and I am also passionate about an old car I’ve had for decades. This programme is up there with those passions.”

Apted, like Turner, has remained emotionally committed. The British director, who went on to make his name in Hollywood, directing films such as Coal Miner’s Daughter, Gorillas in the Mist and the Bond thriller The World is Not Enough, has stayed at the centre of the Up series, returning every seventh year to chart the lives of participants who have now become familiar to television audiences. As children, some of the stars were precocious where others were shy; some were well-off and some disadvantaged. Apted deliberately intended to contrast the opportunities of children of different social classes.

“It was about class in the beginning,” said Turner, “but it was not done scientifically, in the way it would be now. It was quite random: simply going to schools in London and Liverpool and the Lake District.”

He believes that neither class nor their involvement in the Up series has materially altered the lives of the cast.

“They have all had the lives they would have had, I think,” he said. “We’re only there every seven years. A lot of them did not classify themselves by social class as children, of course. Yes, we are an intrusion. But then when they are long gone, their grandchildren will be able to see what they were like.”

Turner has had great control over the way the participants were portrayed from the age of 28. “Until the 42 Up series, it was all shot on film, so I was the only one who saw the footage. I had to compose it, and expose it. There was a huge act of trust that took place.”

For 63 Up, Turner has chosen sequences from the archive not seen for 21 years. “I make sure we don’t always go back to the same stuff.”

The defining feature of the whole series has, he believes, been its relaxed mood.

“Whatever you film with the participants, you are given this one opportunity, but we always want to make sure there is no more stress for them than necessary. Even the way I light it is part of that. I don’t draw the curtains or anything. We want to show their normal lives. They know I have their interests at heart. We want to keep it very conversational.”

That atmosphere of trust remains, he said, despite the banter between the 14 subjects and the ageing crew.

“They are all good at talking now. I know from making other films that finding a way to get people to speak and make sense can be very hard. The familiarity with the team helps.”

But catching up in Australia with Symon and Paul, two orphan boys first filmed at Barnardo’s, Turner was reminded that the participants can still surprise him.

“We were walking through Melbourne Cricket Ground and they suddenly both lit up at this statue of a fast bowler. We didn’t know they both knew so much about cricket. But I had to be there to film it, to get those bits of magic.”

And what of the other 12 participants? Well, the passage of time has brought death and illness to some. “This time we lost Lynn, who died in 2013. It was hard. But we have made a true documentary about these lives that is also a document of the times. The clothes and the buildings all change with the people.”

The most elusive of Apted’s subjects, Neil, has weathered some dramatic turns of fortune, facing homelessness before becoming a local councillor, and at one point being taken in by Bruce, a fellow participant. This time Turner tracked him down abroad.

“I have to be on my mettle to get the right image for each of them. In France this time, with Neil, I happened to see him reading a book outside on his own and I knew we had to film it. There was no coming up to look over his shoulder. We stay distant. It was probably something Neil does every day, but they each need to have their particular images down the decades. We have to get these moments, these vignettes.”

Turner admits the crew do “occasionally set things up”, but only as far as checking out the route that might be taken on a dog walk and choosing the camera angle. The crucial “magic” moments, though, are the result of “being diligent”.

Down the years Tony, a London cab driver, has been one of the most popular and visible of the Up cast. And Turner sees the misfortunes he has suffered since the 2008 financial crisis as evidence of the social history the series captures. “Tony wanted to buy property in Spain and have a bar there. That was his aspiration. But he was hit by the crash and it all fell apart. That’s a tragedy, in a way.”

Turner is in touch with Tony, and with Jackie, childhood friend to Sue and the late Lynn, and she has stayed at his Cheshire home. “We all keep up with them a bit in between,” he said. “They are all very much part of a common bond that is between us. Up is a family.”

And Turner keeps up with Apted, now 77, visiting him in LA. “He is a very good friend and good to work with. When we go out to film, he is never one to say he would like a particular shot. He leaves it to me, but all the crew are careful not to ask the participants any of the big questions, because Michael will do that on camera. This time he does ask them about Brexit, although we thought it might all be sorted by now.”

When and how the series concludes will be up to ITV and “to the will of the people in it”, according to Turner. Will there be a 70 Up?

“It is difficult. We are no spring chickens any of us and, as Michael has said, there is a worry about covering people just getting older.

“I’ll be 80 by the next one. Seven years ago I said I would do the next one if I was able and I feel the same now. I have the same enthusiasm.”

The project has been copied across the world. The Child of Our Time series owes a debt to Granada’s adventurous commissioning. “The only person who has been chronicled as much as these 14 people, recorded at all ages, is the Queen,” said Turner, with satisfaction.