"#Refugees Welcome," reads the sign as you enter New North London Synagogue. Far from being a meaningless slogan, it's a core philosophy for Rabbi Jonathan Wittenberg, who has led the UK's leading Masorti community for nearly 30 years and is at the forefront of those calling for Britain and Europe to do more to help people fleeing Syria.

"It's a moral catastrophe," he says of the situation unfolding and the lack of pan-European agreement. "It's an outrage."

Having visited the Idomeni refugee camp in Greece once and been twice to Calais, he has heard first-hand about the horrors people are fleeing from. "There are terrible accounts of how children are treated. The level of violence is unimaginable to those who haven't been through it."

He echoes calls for a modern-day Kindertransport. "Unaccompanied children who are in danger of being sold into all kinds of modern slavery are the most vulnerable and must be looked after."

The rabbi, an environmentalist, campaigner and blogger who has made a name for himself in and beyond British Judaism for his thoughtful and softly spoken manner, is unequivocal about the need for the British Government to go further. "There is some issue about how many we can absorb and what's fair, but Britain's response has been minimal, really minimal," he says, adding that, if he wasn't a rabbi, he would be campaigning on this issue full-time.

His commitment to aiding refugees, which he makes clear is shared across the spectrum of the Jewish community, is something he sees as a moral imperative, for him personally and for everybody. And while he is clearly someone who carries the weight of the world on his shoulders, his commitment to refugees is perhaps unsurprising in the light of his family history.

The son of escapees from Nazi Germany, Wittenberg grew up in Glasgow in the shadow of the Holocaust, aware that many in his extended family had not been so fortunate. But it wasn't until nine years ago, when an aunt in Jerusalem died, that he learnt the full story of what happened to his paternal relatives.

As he recounts in a new book, My Dear Ones, he was clearing his aunt's Rechavia home when he opened a forlorn-looking trunk containing a series of letters. Written in German, much of the handwriting undecipherable, the dates jumped out at him. They were from 1938, 1939 and 1941, postmarked Germany, Czechoslovakia, Poland, New York and Jerusalem. "I realised that these were a treasure of family history."

The correspondence, which flowed regularly between the relatives both before and at the height of the war, documents starkly how lives and communities were shattered as the Nazis tightened their grip. They reveal, in heart-breaking detail, how those still under Nazi rule, including the rabbi's great-grandmother Regina and great-aunts Sophie and Trude, experienced endless brutality and yet remained optimistic. "When peace comes I'd love to come and visit you… hopefully it won't be too long," wrote Sophie in 1941 to her brother in America.

The letters also focus on efforts to secure Regina papers for Palestine, so much so that the book occasionally reads like a thriller even as you know the devastating outcome. "There's the maddening frustration of the right papers coming at the wrong time, and she can't use them," agrees Rabbi Wittenberg.

Writing it was naturally harrowing, although, as he points out, being second-generation, he did not have to see the faces before him. "For my father's cousin in New York, there were letters she couldn't bear to sort because she knew the faces."

The book, says Rabbi Wittenberg, is a tribute to those lost in the Holocaust, and especially to his father, Adi, who died shortly after the letters surfaced. He writes movingly of his father's death in the book, explaining his regret about not asking more questions earlier (Wittenberg's mother Lore died when he was five, and his father then remarried Lore's sister, who became a much-loved stepmother to Wittenberg). His wife, Nicola Solomon, general secretary of the Society of Authors, has read the book. His children are keen to do so. "My son said to me: this is my family in here."

"I was moved by a sense of uncovering and learning about people who had just been names," he explains. He made a point of visiting the key places, including Holešov (Holleschau), where Regina and Sophie lived during the early war years and where his great-grandfather served as rabbi for two decades - and Ostrów Lubelski, from which Trude and her family were taken to their death.

At the latter, he saw the empty field that once housed the Jewish cemetery, a pile of broken gravestones in the centre. His aim now is to erect a memorial at the site. His dedication is such that I don't doubt he'll achieve it.

Wittenberg, a close friend of the late Holocaust historian David Cesarani, sought both to record his family's testimony but also to contextualise how Nazi policy developed differently in different places. "David said lots of people have got letters but not many people have a comprehensive series like that," he explains. "The letters allowed me to use the family to tell the narrative of much of the Holocaust, including British politics, the Yishuv, but above all Nazi politics, so that the reader will be able to read it as history as well as testimony."

He is loath to draw a direct comparison between the situation then and now - "history repeats itself but not exactly" - but suggests there are parallels, not least with the 1938 Evian conference. "Goebbels scored a propaganda victory by saying 'you complain how Germany treats its Jews, you don't want them either'", he notes. And although he rejects doomsday prophesies that we are living through a rerun of the 1930s - "I don't see the Hitler figure and his henchmen" - he considers these dangerous and frightening times, particularly for Jews in Europe.

"There's the rise of the far right and xenophobia, and a new kind of violence that has no respect for any kind of life whatsoever," he says. "There's also the nefarious narrative being spread about Jews, which sometimes goes beyond legitimate criticism of Israel to express antisemitism.

"We see it on the far left and on the far right, this misunderstanding, this lack of sensitivity, lack of facts. It's very dangerous."

In particular, the father of three, who himself attended Oxford University and then Goldsmith's College, is troubled by events on campus. "Jews and Jewish societies feel themselves cornered and alienated," he says. "Campuses are not all the same but students are often in the front line of attack not just because of Israel but sometimes just for being Jews."

So do we have a future in Europe? "I very much hope so. Jews have made a huge contribution to Europe, Europe offers models of democracy which have been beneficial to Jews and it's good for Judaism to have both Israel and a strong diaspora. But nobody knows how things turn out and things feel very concerning at the moment."

Talk turns to Ken Livingstone's "gratuitous, nasty and erroneous" claim that Hitler was a Zionist "before he went mad". Rabbi Wittenberg addresses the history of Nazi efforts to get Jews out of Germany in My Dear Ones. "The history is quite complicated, but to call Hitler a Zionist is perverse. Hitler hated Jews, his initial aim was to get them out of Germany any way he could and later when he couldn't because of the failure of the Russian front, he wanted them dead."

Despite the Ken effect, support for the Labour party did not plummet in Barnet and Camden at the mayoral election. But there are signs that nationally the picture is different, and a JC poll has suggested Jewish support for Labour has fallen to 8.5 per cent.

In his view, the party does have an antisemitism problem. "People are saying to me in my community 'I've always voted Labour, but now I feel I can't'," he says. He backed Labour Sarah Sackman in Finchley last year (although he offers enormous praise for Mike Freer, too) but admits, dejectedly, that he doubts he could back Corbyn's Labour.

Still, he is cheered by Sadiq Khan's victory as mayor, including the choice of his first engagement as a Holocaust memorial service.

Rabbi Wittenberg expresses hope this will offer an opportunity to build better bridges between the Jewish and Muslim communities - "indeed between all the communities of London". He has been a tireless champion of interfaith engagement throughout his career and is adamant that this must continue, with the church as well as with the Muslim community.

But, Khan aside, how realistic is this bridge-building, given the anti-Israel and even anti-Jewish "narrative" that he mentioned? "The interfaith scene is thriving, but how deep does it go, who does it reach," he wonders. "It is, unquestionably, a difficult time. It's very difficult to have subtle, nuanced conversations about Israel and about Judaism and interfaith. It's pushing people into corners in a very unpleasant way."

Despite this, he says we must persevere; go into schools, work with churches and mosques and temples. "It really matters where we can to turn enemies into friends or at least have an open conversation."

We meet at New North London, which, even on a sunny weekday evening, is a hive of activity. Children are leaving barmitzvah classes, congregants of all ages are arriving for a shul event . We could chat for hours but communal duties call; his busy-ness and his commitment to his congregation is evident (he jokes that rabbis don't get much downtime; what little he has is spent gardening).

Ever modest, Rabbi Wittenberg credits the other rabbis, along with the professional and lay teams, for the community's success, although after nearly three decades at the helm, much is down to him.

Since he joined as rabbi in 1987, Masorti Judaism and the Noam youth movement have grown and thrived. It is a point of pride there is now a generation of Masorti-trained rabbis with roots in Britain. He is cheered, too, to see so many doors into Judaism today, from Tikun Olam to Limmud, which he credits with "making Jewish learning cool.

"There's a strong sense of future. There's strong, young leadership which wants to be knowledgeable, committed and disciplined in their Judaism," he says. At the same time, he fears some people take Judaism for granted and undervalue the benefits of the community. "We need each other, we need to care for each other through life's journey. We also are heirs to a rich, complex and beautiful tradition that deserves better than to be neglected."

By and large, he is optimistic. "There are so many people in the community who do things I admire," he says. "I feel often I'm led by my community and the values I see around me. I'm grateful for that. The response to refugees is just one example of many."

'My Dear Ones: One Family and the Final Solution', by Jonathan Wittenberg, is published by William Collins