Two and a half years ago, Michael Winterbottom began a sabbatical. He had been bruised by losing control of the final cut on Greed, his satire about a Philip-Green-alike, played by Steve Coogan. He felt the studio had sabotaged the film’s impact by neutering its message. Winterbottom wanted captions over the end credits, explaining how much real-life fashion tycoons are worth and how little they pay their workers. The head of Sony did not. The truth, it seems, could not be handled.

Winterbottom’s sabbatical did not last long, but his commitment to facts over fiction has only strengthened. First, he wrote a book, Dark Matter: Independent Filmmaking in the 21st Century. It featured conversations with fellow British film-makers about the realities of the industry – in transcript format, with no space for misinterpretation.

Then he started work on This Sceptered Isle, a mini-series about the first six months of the pandemic in the UK, told through the experiences of real people. It is due on Sky in September. Kenneth Branagh plays Boris Johnson. Winterbottom is worried they have been a bit soft on him – hindsight, in this case, has been quite the gamechanger.

But, he stresses, the show is as straight and unspun as you can get. No new revelations, no radical speculation. “It’s very neutral. Almost a diary. A record of something we all lived through,” says Winterbottom over a video call.

Now comes the release of something structurally similar. Eleven Days in May is a documentary about the children who were killed in Gaza during Israel’s 11-day bombardment last May. At least 60 of the almost 200 Palestinian victims were children, but the exact figure is impossible to verify. The film opens with BBC News footage of the airstrikes – the type viewers are familiar with, “but then tend to forget about,” says Winterbottom – before running through the conflict chronologically, telling the viewer about the children who died each day, via interviews with 28 families.

Kate Winslet provides the voiceover, Max Richter the soundtrack. Neither do more than they need. It is spare, respectful and overwhelming.

“We wanted to keep it as simple as possible, almost like a photo album,” says Winterbottom. “The format wasn’t intended to give dramatic or emotional shape. But you hope it gradually accumulates some sort of emotional power – and gives a fair moment for each family.”

Winterbottom had the idea for the film and edited the footage, which was shot in liaison with Mohammed Sawwaf, a local film-maker, to whom I speak, over a video and through an interpreter in his office on the Gaza Strip.

It is hard to hear over the constant clatter and traffic and endless internet blips (emailing the rushes to the UK was an epic undertaking). This building, says Sawwaf, gesturing around, is one of the few on the block not yet razed and rebuilt.

“Initially, most families refused to take part,” Sawwaf says. “It needed a great deal of persuasion.” This was done by the families consulting each other and Sawwaf telling them: “Your child is not a number. You should show the world that these are people who had aspirations and who ceased to exist.”

In the film, we don’t hear or see Sawwaf, only the bereaved relatives, including prolonged shots as they compose themselves before speaking: parents and grandparents facing down the lens; children shifting uncertainly, sometimes giggling, sometimes wiping away tears.

Sawwaf was amazed by the resilience of the parents: “The refusal to remember their children in a dark light. If, God forbid, I were in their shoes, I wouldn’t have the courage to talk.”

They seem remarkably free of anger, I say. “These people are by nature kind and welcoming,” says Sawwaf. “They’re talking about their children lost to violence. They appreciated their lives being commemorated.” He pauses. “But, also, the deaths have cost them a great deal. They feel broken down, so exhausted by the grief they couldn’t show much anger.”

The most upsetting thing to ask about, it turned out, was not the circumstances of the killing, but rather the hopes held before it: “‘What were they dreaming of? What did their future hold?’ This specific question was much harder,” says Sawwaf.

What was left out? Not a lot, he says. Few of the families agreed to be interviewed at the site where their children died. His crew filmed a few visiting the graves, but it generally just felt too much, says Sawwaf. Sometimes, the recollections overwhelm the loved ones. One brother stops, sobbing. Two mothers separately speak of not being able to comprehend what has happened. Every noise outside the door, they think, might be their son strolling back home.

For western audiences, the most shocking moments are likely to be the photos of the dead or mortally wounded children, which are interspersed with photos of them in happy times. These indelible images – as well as social media footage of them being taken to hospital or put into the ground – were released by the families, says Sawwaf, some of whom had already made their own videos about the killing.

The purpose was simple: “The contrast between life and death. To show people what war did – it killed a future.”

Winterbottom says he thought a lot about whether to include the photographs of dead or dying children. Months submerged in the footage taught him that the “relationship to loss and grief” in Gaza is “definitely a different concept to that in the UK. More public. It’s a more visible aspect of loss and remembering.” Most families have put up enormous posters commemorating their children outside their homes.

Had the film been about children lost in England, says Winterbottom, it is unlikely that their families would have consented to the use of such unflinching photographs – or even possessed them in the first place.

“Ninety-nine per cent of the film is people talking about the things they loved about their child,” he says. “But I think it should be shocking, when you see a child killed by a bomb. We ought to be shocked that children are killed like that.”

Church bells ring out behind him. Winterbottom is in Italy, scouting locations for a film he hopes to make later in the year, set in Tel Aviv. (Filming in the city would be too challenging.)

He says he is haunted by the random nature of the deaths: some in the street, some at home, some asleep in bed. “That sense that you can’t protect your children, whatever decision you make. That lottery: if you’re unlucky, something terrible is going to happen. That sense as a parent must be awful.”

In the final story in the film, a mother tells of feeling desperately trapped even before the bombardment, of hoping to leave, and then of sending her daughter on what turned out to be a fatal errand.

“It must be absolutely terrible to have not only the loss of your child, but that in the back of your mind as well,” says Winterbottom. He has three children: two grownup daughters and an 11-year-old son. His mother lost her firstborn as a baby, he says, through illness. “For 50 years, it was always a presence in the family. You never get over those kinds of losses.”

When I speak to Sawwaf later the same day, over the fuzz and chaos, I ask him what someone like me could never understand about living in a war zone. Both he and his translator look defeated. “It kills any ambition. Any thoughts of security or settling down to lead a normal life. It drains you. It’s really dreary.”

And it is worth remembering, Sawwaf adds, that while those caught up in other wars “might have the chance to seek refuge in other countries until it’s over”, not only is that not an option for most Palestinians, but also “there is not a single shelter in the Gaza Strip – no safe place – although it has been besieged for 15 years”. He sighs, approaching wryness. “It has its own negative uniqueness.”

Winterbottom, too, seems wary of comparisons with Ukraine – at least those not tempered with scepticism. “A lot of countries are making a lot of effort to try to stop what’s happening in Ukraine: welcoming people from the country, trying to put sanctions on Russia.

“But in the first 20 years of this century, we have bombed Iraq, bombed Libya for regime change, which is illegal; we haven’t put sanctions on Israel when they bomb Gaza, we don’t put sanctions on Saudi Arabia when it bombs Yemen. But if you’re a family losing a child in Ukraine or Gaza or Yemen or Iraq, it’s the same thing.”

He is frank, sad and straightforward. His message has not been muddied this time. When he was raking over the fallout from Greed, he told me he had wanted “to make people feel angry and frustrated and to want change”.

Eleven Days in May is an act of remembrance that also acts as an unmediated and effective assault on its audience. It is sombre cinema that can’t help but alter your comprehension of conflict. “We want people to see that war brings only destruction,” says Sawwaf. “War is not a solution.”

One group of people have neither the need nor the desire to see the film: the children’s families. There has been no screening in Gaza. “Not yet,” says Sawwaf. “It might reiterate the sadness, so they are a bit reluctant. But they ask me about the impact elsewhere every day.”

• Eleven Days in May opens in Picturehouse Cinemas on 6 May. A premiere hosted by Russell Brand will take place at Picturehouse Central in London on 4 May to raise money for Hoping Foundation