The answer lay in the goal mother and son were each striving towards - a goal that formed the most common subject matter in the letters and one that became increasingly urgent as time passed: Kurt’s escape.

So why did she seem to urge Kurt continually to try to see his father in the face of evidence of intimidation and rejection?

With nowhere to turn and dwindling financial reserves that also meant his clothes were disintegrating, Kurt writes on 24 May that he would be seeking out a park bench to sleep on.

I was on tenterhooks as I waited for the final instalments of letters from the transcriber.

Kurt desperately needed to get a passport. Without it, any progress his mother made would come to nothing. The problem was that he still needed his father’s permission to travel. Hedwig encouraged Kurt to stay in contact with him to lay the groundwork for the request, but as I had discovered, their relationship was going from bad to worse.

By 30 April 1939, Hedwig’s employer Mrs Seaton had agreed to help by writing to the Kindertransport committee, sponsoring Kurt and finally setting about finding him a place of work. However, it was two months and many anxious letters before a possible candidate, a farm, would be found.

She was arguably better placed to do this when based in the UK. But it must have been exceedingly difficult with a full-time job, living in rural isolation and not speaking a word of English.

If there was a plan, it was to try everything and anything to extract Kurt. From the beginning of her exile, Hedwig had been trying to get him a work-based visa and the kind of financial sponsorship necessary for this, or a place on a Kindertransport as her dependant.

Hedwig with her five siblings and some of their spouses. Sister Helene Kompert (5th from the left) and Ella Müller (8th from the left) both died with their husbands in the Holocaust. In the front row is Hedwig’s mother and Kurt with a sling.

Kurt’s father’s blunt refusal to talk eventually gave way to a promise to sign the relevant papers at the guardianship court after Kurt cornered his father at work. Kurt turned up at the court alone on 9 June full of hope, to find that his father had lied. “This low-life has of course not signed…my father doesn’t care about me,” he writes in a letter on the same day.

An intervention from aunt Otti’s non-Jewish husband Stefan was decisive, but only after he was told that it was “shameful” that an Aryan like him should represent the Jewess Hedwig. Kurt’s father agreed to sign a document that absolved him of any further parental duties or school payments, meaning Stefan could authorise Kurt’s passport.

“A heavy load has just been lifted from me,” writes Kurt on 25 June, and in happy anticipation of a reunion: “I think that you will recognise me: I have become a bit bigger and more manly but don’t yet look like Clark Gable.”

Now the burden was entirely Hedwig’s.

The farm placement had not yet materialised and Kurt pressed his mother to get an answer. On 28 June, a breakthrough made it unnecessary. The Quaker Society of Friends, a key Kindertransport organiser, told Kurt that he would be travelling on 11 July, but only if he could get all of his documents together. Otherwise, the next available place was mid-August, just two weeks before the war that he sensed was coming would start. “I am overjoyed,” he wrote to his mother when the news arrived.

Kurt raced to comply and on 10 July, just one day before his scheduled departure on a Kindertransport, he got his passport. Having already endured parental separation, he was in the unusual position among his travelling companions of steaming towards a reunion.

Most of the other children on the train, along with the almost 10,000 who made the same journey to safety on Kindertransports before war broke out, had, unbeknownst to them, said their final goodbyes before boarding.

With the benefit of hindsight, leaving first was absolutely the right decision. Had Hedwig stayed, Kurt may not have found a place on a Kindertransport, although as a Mischling he would have been spared death, if not discrimination, in Austria.

Wherever he was, Kurt would have been even more alone in the world because Hedwig would have faced the fate of two other sisters who died in Nazi camps.

If at the outset of reading the letters I had expected to learn why Hedwig and Kurt may have found it hard to talk about this period in their lives, then by the end of the 100 pages I had unexpectedly discovered why Kurt may have avoided talking about his father, too.

Hedwig had little choice for a decision that at first seemed callous, and her devotion to Kurt shone through in the letters. But the opposite could be said of my grandfather. I was troubled by the discovery that he was responsible for a great deal of the trauma and almost prevented the reunion.

To cap it all, he had become sympathetic to the hate-filled ideology that takes the ultimate blame for Hedwig and Kurt’s predicament in 1939.

But my most lasting impression from the letters is how an unshakeable bond between mother and son helped them to prevail in dark times so that they could be together again.