Unless food is the focus of a film, it rarely comes up in conversation with the director, but with James Kent, 51, it was inevitable. His great-grandfather was Lloyd Rakusen of matzah-making fame and it happens to be my favourite snack. "Mine, too," says James enthusiastically. "They are just so more-ish and delicious with scrambled eggs. Forget seven days - at Passover, a van would arrive at our house with enough matzah to last six months, but I never tire of it. "

We're not here to talk unleavened bread, however, but discuss James's first major cinema production, Testament of Youth, which is released today. Based on the best-selling memoirs of Vera Brittain who lost her fiancé, brother and two close male friends in the First World War and starring Dominic West and Hayley Atwell alongside newcomer Alicia Vikander, James was chosen to direct because he has made everything from feature-length documentaries about Chopin and the murder of Gianni Versace to the BBC drama series The White Queen. "It isn't easy to convince people to give a big budget (£7m) to a debut director but, in the end, the backers listened to the producers who saw an emotionalism and gritty authenticity in my work that I could bring to Vera Brittain's story," says James. "It is a story about loss, valuing humanity and the pricelesness of a single human being. We have to deal with the death of a loved one and we have to pick ourselves up from it in order to become richer, rounder human beings, which is what Vera did. It's about living, loving and losing."

Researching the period in which the film is set was instinctive for the documentary-maker who read volumes about the First World War along with Brittain's diaries. "You want everything to sound and look authentic," says James, who could also rely on Vera Brittain's daughter Baroness Shirley Williams, the Liberal Democrat peer, to tell him all about her mother. "She was very involved and invaluable when it came to advising Alicia about how Vera behaved. Her presence authenticates the film." Though he credits his cast for the film's success to date, it is James' insight and understanding of loss that gives it depth and he drew inspiration from the loss of his own family - nine great aunts and uncles - in Auschwitz during the Holocaust.

"It was only when I came to do a family tree for my brother's (producer Nick Kent) 50th birthday that I learned the story," he says, though the enormity of his discovery did not strike home until he spent a month at the concentration camp in 2004 while making Holocaust: A Music Memorial Film From Auschwitz. Made to mark the 60th anniversary of the camp's liberation, the award-winning film brought together a group of internationally renowned musicians who performed on the site and gave their survivors' testimonies.

"It was a long time to spend there and eventually I knew every inch of the camp," says James. "But I waited until the last day to go to the back of the barracks and there I burst into tears. I had kept it all bottled up until the end as it was the only way I could get through it."

Though his family were responsible for making and mastering one of the most significant contributions to the Seder table from a factory in Leeds, James describes them as being religious in a "British liberalised way".

Living on Great Portland Street which is handy for Soho's edit suites, James also visits the neighbouring synagogue on account of the cantor's beautiful singing. He is also a reliable source if anyone needs a box of Rakusens as there is always one or two in the cupboard.