Journalists on national newspapers get used to crank calls from people claiming the government is controlling their minds using radio waves or the Duke of Edinburgh is opening their post. So when Val Hann first called me at the Observer almost exactly five years ago, I was, I have to say, extremely sceptical about what she told me.
She had read an article I had written about Unity Mitford, the 1930s society beauty who became a groupie to Adolf Hitler and shot herself in Munich at the outbreak of war. Although the bullet entered her brain, Unity survived and lived out the rest of her short life as an invalid. But my caller claimed to have an extraordinary new angle on the story.
Val was a little nervous as she explained that her aunt Betty Norton had run a maternity home to the gentry in Oxfordshire during the war and that Unity Mitford had been one of her clients. Her aunt's business, in the tiny village of Wigginton, had depended on discretion and she had told no one except her sister that Unity had had a baby. Her sister had passed the story on to her daughter Val.
I casually asked who she thought the father might be and there was a short silence on the other end of the line before she said: "Well, she always said it was Hitler's."
I must say I was tempted at this point to put down the phone. Christmas was coming and I was very busy. But for some reason I decided to carry on listening to this bizarre tale. Val didn't sound mad, and she said she was merely passing on a family story.
The child was a boy, she believed, and he had been given up for adoption. She didn't want any money; she just wanted me to look into it. So here was the prospect of Adolf Hitler's love child alive somewhere in Britain - it was either the scoop of the century or completely bonkers. But it had to be worth a few hours of my time, even if it turned out to be a dead end.
My original story had cast doubt on the official version of events about Unity Mitford's return from Germany. In the millions of words written about the Mitfords, accounts of Unity's movements in those early months of 1940 remain sketchy. And, despite the obvious trauma to the family, only a handful of the hundreds of letters that the letter-writing sisters have had published discuss this period.
The newly released diaries of Guy Liddell, number two at MI5 during the war, suggested that the security service was not even convinced Unity had shot herself in the head. Liddell was determined that Unity should be searched and interrogated on her return from Germany and then interned for her Nazi sympathies.
Writing on 2 January 1940, Liddell made a powerful case. "Unity Mitford had been in close and intimate contact with the Führer and his supporters for several years, and was an ardent and open supporter of the Nazi regime. She had remained behind after the outbreak of war and her action came perilously close to high treason. Her parents had been associated with the Anglo-German Fellowship and other kindred movements, and had obviously supported her in her ideas about Hitler.
"We had no evidence at all in support of the press allegations that she was in a serious state of health and it might well be that she was being brought in on a stretcher in order to avoid publicity and unpleasantness to her family."
However, Liddell failed to convince his superiors and the home secretary himself, Sir John Anderson, finally intervened to say that nothing should be done on Unity's return. In fact, Liddell was wrong about her injuries. She had indeed shot herself and later died of an infection caused by the bullet in the brain.
Nonetheless, it still seems astonishing that she was never questioned, considering how close she was to Hitler. As Liddell wrote at the time: "If we had been dealing with Miss Smith or Miss Joyce, the probability was that we should not be arguing the case."
If it hadn't been for Wigginton, I would never have taken it any further. Val gave me an address for the maternity home, Hill View Cottage, and I contacted the present owner, who agreed to show me around. She confirmed that Nurse Norton had indeed used the cottage in her work as a midwife. She also agreed to introduce me to the one person in the village who remembered Unity being there. Audrey Smith was a little girl at the time, but by pure chance her sister (now dead, unfortunately) had worked at the home and had talked about Unity. Audrey herself claimed she had seen Unity wrapped in a blanket and looking very ill. However, she insisted that she was at the home not to have a baby, but to recover from a nervous breakdown.
By now I was intrigued and wrote to Unity's surviving sister, Deborah, the Duchess of Devonshire. She had been angry at my original article and had written a furious letter to the Observer denouncing Liddell's claims that her sister might not have shot herself. She also suggested I take less notice of the gossip of villagers. The Duchess of Devonshire was adamant that there was nothing in the Wigginton story and claimed she could, if necessary, produce her mother's diaries to prove it.
At this point I decided to return to the National Archives, where I discovered a file on Unity that had been sealed under the "100-year rule" - reserved for only the highest classification of top-secret files. An official told me that it was possible to have the classification of such files reviewed and I applied to have the file opened. To my great surprise, the Home Office agreed. Inside was a startling new piece of information: it wasn't quite the birth certificate of a child, but here was hard evidence that Unity might not have been quite the invalid it was supposed.
By October 1941, while she was living at the family home in Swinbrook, Oxfordshire, the police picked up rumours that "Unity Mitford has formed an attachment for an officer in the RAF". Further investigation found that she had been "consorting with Pilot Officer John Sidney Andrews, an RAF test pilot".
As a result, Andrews, married with a child, was transferred to the far north of Scotland.
At this point, the trail went cold. There were too many loose ends for a news story and my research sat in my notebooks until this year when I mentioned it to Mark Roberts, an executive from Channel 4, who agreed to put the story on film. Further research, including an exhaustive trawl through birth records at the Oxfordshire register office, confirmed that Nurse Norton had helped dozens of wartime mothers give birth at her maternity home. But no record of Unity Mitford. Airman Andrews, it turns out, was a former bank clerk, and died in a Spitfire crash in 1945. There is no evidence that he ever saw Unity after his transfer to Scotland.
So what is the truth about Unity Mitford's missing months? Is it possible that the sightings in Wigginton were a case of mistaken identity? Or was she there to recover from a nervous breakdown? One woman still alive who could add to the story is the Duchess of Devonshire, formerly Deborah Mitford, who travelled back to Britain with Unity in 1940. But she has so far declined to be interviewed for the programme. She has also told people close to her that any suggestion of a child is fanciful.
Five years on from that original phone call, I have taken this story as far as I can. It remains a mystery and I remain as sceptical as I was when I first spoke to Val Hann. But one nagging thought remains: if Unity Mitford was in Wigginton during the war, what was she doing at a maternity home?
"Hitler's British Girl" will be shown on Channel 4 on Thursday 20 December at 9pm