28/01/2015 07:42 GMT | Updated 30/03/2015 06:59 BST

Why It's Important to Document the Lives of Holocaust Survivors

It was whilst I was having afternoon tea on a sunny day in a small English town, with Concentration Camp survivor Kitty Hart-Moxon that she first asked me if I wanted to see her Auschwitz number. Expecting her to roll up her sleeve and show me a tattoo on her arm, I was taken aback when instead she went to a glass cabinet in the corner of her sitting room. After rifling amongst the ornaments, she brought out a small sealed glass container filled with a clear liquid and two pieces of what looked like torn plasters mounted inside.

"The top one" she said, "was my Mother's number. The bottom one was mine". It took me a moment to realize that what I was actually looking at were pieces of skin, each marked faintly with a series of numbers. She explained that she'd had hers cut from her arm by a surgeon to avoid awkward questions about what it was, and that after her Mother's death she'd asked the coroner to remove hers too. She'd had them mounted together in formaldehyde as a lasting reminder of the two years they'd spent in the Nazi's most notorious concentration camp.

I've been a film-maker at the BBC for over two decades, and I've been privileged enough to witness many extraordinary moments, but this was one I will never forget. I was visiting Kitty to begin making a film to mark Holocaust Memorial Day, which this year also marks the 70th anniversary of the liberation of Auschwitz. And what struck me vividly in that sunlit room was that whilst Kitty had physically cut the mark of Auschwitz from her body, she couldn't get rid of it. She couldn't throw it away. Surely such a reminder would be best destroyed? And yet as I got to know her during filming I realized that tiny piece of frail skin symbolized a memory that Kitty fervently believes should never be forgotten. Yes, it's the physical evidence of the barbarity man is capable of. But it's also a warning for future generations to stand up to the tyranny of those who would dehumanize. Those who would reduce people to numbers like cattle. Those who would slaughter in the name of an ideal.

It's how and why the memories of one of the darkest moments in human history should be kept alive that formed the theme of the film. And during the months making it I was struck by the myriad of ways those who suffered the atrocities of the Holocaust have chosen to pass their memories on. How they refuse to allow the echo of what they witnessed fade. I found myself moved to tears in a wood in Lithuania. All around me, carved in the trees, were the haunting figures of men in chains and women clutching their babies. They were the work of Jakovas Bunka, the last remaining Jew in the small village of Plunge. One thousand seven hundred of his neighbours, family and friends were slaughtered here in 1941, and buried in mass graves amongst the trees. Jakovas died two weeks after we filmed with him. But his memories will live on. His pain is carved into the wood to ensure the echo of what happened here never fades.

I travelled with Kitty to Belsen some months later and I think it was only here that I fully grasped just why the memories of those who suffered at the hands of the Nazis should so powerfully endure. In 1945, as the Nazis' grip on power was beginning to wane and the Allies were closing in, Kitty and her Mother were taken to a railway siding outside Belsen. As they approached they saw dozens of trains - sealed cattle trucks - and from inside they heard people crying and screaming. More chillingly some were silent. She and her Mother were pushed inside a truck and the door was sealed. Immediately she realized there was no air and that very soon they would suffocate.

We filmed Kitty as she returned to a siding near Belsen, and entered a carriage erected there as a memorial to those who died. As she looked around her, she said "The Nazis believed you mustn't have any survivors because they thought survivors are going to tell the truth about what had happened, and they didn't want any witnesses."

Thousands survived the death camps, only to be killed in these railway carriages as the Nazis panicked in the light of the Allied advance. It was here that they tried to eradicate all those who could speak of their murder. But Kitty didn't die. Her carriage randomly moved and some hours later the door was opened. She was taken to another camp which, a week later, was liberated by the Americans.

Kitty's survival and her determination to pass on her memories to a new generation denies the Nazis their last wish. The memory of man's capacity for evil won't be forgotten. It lives on as a warning for future generations in Kitty's stories. In her memories. In the tiny pieces of skin that sit in a small corner cabinet of a suburban house in Hertfordshire.

The Holocaust: A Story of Remembrance is on BBC One tonight at 10.45pm