"I can’t really communicate with others properly, because they don’t know what I’m talking about. I mean, how many people in England have had their parents murdered or seen a gas chamber in action? It’s just different. It has affected me, yes.”

Frank Bright, now an old man living in Suffolk, has rarely talked about his childhood before. As he spent some of it in Auschwitz, where his family were murdered, this is understandable.

Child survivors are now pretty much the only ones left to bear witness to the industrialised extermination of European Jewry, so the stories told in the BBC2’s extraordinary documentary, The Last Survivors, are those of bewildered and traumatised children who have grown old carrying the burden of the past with them.

Some, like Bright, have found it difficult to talk at all. Others have struggled with sharing their experiences with their own children.

Some — like Susan Pollack —have told their story again and again, revisiting painful memories to educate future generations. Even she keeps her emotions in check. “I haven’t been able to cry,” she tells the film maker Arthur Cary, “because I think crying would have no end.”

Cary, who is not Jewish, admits he felt “intimidated” when he was commissioned to make the film. His method seems simple — talking to, and spending time with as many survivors as possible, looking to make a connection with them.

He told the audience at a screening at JW3 that he found they were “very normal people who have lived through extraordinary times.”

Of the dozens that he spoke to, some appear in the film just as faces, others say just a few words. He picks out the stories of seven as a focus.

There was a painful moment at the screening when he reassured a woman whose visit to a school had been filmed but not included, that, yes, it could have had its place in the film, and it had been a very difficult job to decide what to leave out. He notes the “quiet dignity” of the survivors.

The film is long at 90 minutes, but one struggles to see how it could have been shorter, given the breadth and depth of its ambition and schievement.

Alison Kirkham, Controller of of Factual Commissioning at the BBC, the department which commissioned the documentary, told the audience at the screening that her mother’s family had been torn asunder by the Holocaust.

The next generation are part of Cary’s remit, he wanted to examine the “ongoing legacy for their families,” in part because so many survivors are now in their nineties. In the film survivor Ivor Perl reluctantly returns to Auschwitz to show daughter Judy and granddaughter Lia where he was taken as a boy. Judy expresses her need to share her father’s emotions, to see his tears that never fall.

She feels that she can only get rid of her demons if she can somehow release her father’s grief. This is painfully teased out in a conversation that ends with a hug. In this moment — and throughout the film — the viewer is grateful for Cary’s decision to avoid background music. There is no need here for intrusive violins, telling you how to feel.

Music helped cellist Anita Lasker-Wallfisch survive, when she was picked for the Women’s Orchestra of Auschwitz.She and her daughter Maya also discuss the impossibility of making the connection that the child needs but the parent avoids.

“She did project into me this feeling that there was something wrong with me…why couldn’t I be grateful that no one was trying to kill me?” says Maya. “I was a problem to them because I can never see what people need for their happiness,” says her mother.

"I have provided what was necessary for their survival."

Later in the film Lasker-Wallfisch is shown addressing German politicians at the Bundestag. “Hate is poison, and ultimately those who hate poison themselves . We can only hope that you win this fight.

The future is in your hands,” she tells them. It’s one of few references to the current political climate, a decision made by Cary because he wanted the film to be relevant “one hundred years from now”. He worried that too many parallels to modern politics would date the film. Lasker-Wallfisch — whose tough, abrasive directness may or may not be a legacy of her ordeal — has no such concerns.

Sitting, smoking a cigarette at the Berlin Holocaust Memorial she praises Angela Merkel’s decision to allow refugees into Germany, but points out that some Germans “couldn’t take it,” and that once again “fascists” are being elected.

Some of the most moving scenes come when Manfred Goldberg tells the story of his little brother Herman, who disappeared one day in the concentration camp where Manfred and his mother worked as slave labourers. For decades Manfred nurtured the tiny hope that one day they might be reunited.

Accepting an invitation to Kassel, the town where the family once lived, for the dedication of stolpersteine — small memorial plaques, set into the pavement — outside their old home, felt like the end of a very long journey. At last his brother’s life and death had been publicly acknowledged, at last he could say Kaddish for him.

“Until now we all knew the facts but it has now been publicly and officially, incontrovertibly and indisputedly confirmed. “ He wonders though whether anyone will notice the plaques. “I have my doubts. People become immune to these things very quickly.”

There is hope and happiness amongst the grief and anger. Wallfisch-Lasker contemplates the huge gravestones of Berlin’s memorial to the murdered Jews and reflects that it should have been a garden, to show that not all Jews died.

Some of the survivors interviewed are artists, who process their anguish through their work. Others dedicate their lives to telling their stories. Many have built new families. Their Judaism isn’t much discussed, but it is clear that some came through with faith tested yet intact. And the moment when Susan Pollack dances around her living room is pure joy.

Frank Bright has spent years trying to trace the fate of every one of his old classmates. And he has restored an old air shelter in a Suffolk air field, so children can see what it was like. Sitting in the dark, underground space seems to bring him some peace. “I’ve done something useful,” he says.

“Wasting time is not on. There’s so little time left.”

The Last Survivors is on BBC2 on January 27 at 9pm.