Image caption Ruth Wilson played her real-life grandmother, Alison Wilson, in the BBC drama Mrs Wilson

BBC One's Mrs Wilson - in which the actress Ruth Wilson plays her own grandmother - has shone an unforgiving light on the author, spy and bigamist Alexander Wilson. But not one living soul knows the whole truth about Alexander - my grandfather - except maybe inside MI5.

When the many strands of Alexander's story began to emerge a decade or so ago we joked that it would have to be made into a film one day.

And we turned to my sister Ruth and said: "And you can play Granny!"

And she did.

In fact she went further, making our grandmother Alison the central character in the drama - which became a three-part series in the end, rather than a film.

It may not have been the obvious choice to focus on Granny, but the decision was fine by me because the gruelling poverty and psychological torture Alison endured as a result of Alexander's actions and lies deserved equal billing with his exploits.

At a preview screening last month, about 50 members of my extended family gathered to see the story brought to life.

At the end we were left stunned to silence, many in tears, at what felt like the culmination of our shared past.

But there is still more to discover.

Warning: Spoilers below

After he died in 1963, the fabrications Alexander - known as Alec - had built up during his 22-year "marriage" to Alison quickly collapsed.

He was still married to his first wife - there had never been a divorce - making Alison, as well as Alec, a bigamist.

The country manor he had promised they would inherit was a sham, along with his aristocratic lineage.

It would eventually emerge that he had two further families, with seven children in all.

Image caption Mrs Wilson showed Alec, portrayed by Iain Glen, being recruited as a British spy in India

His credentials as a cad and bounder seem beyond dispute. But they are disputed by his children - each one of whom remembers him as a devoted dad.

"He was very kind, very generous, very loving," recalled Mike Shannon, Alec's son from his second marriage.

But Mike was victim of the cruellest deception in the whole story.

He was told at the age of nine that his father had been killed in the war, when in fact he had moved across London to set up home with my grandmother. He only found out the truth six decades later.

What makes Alec's story so compelling is the maze of contradictions, where it is difficult to grasp what is truth and what is not, let alone what his motives were.

"His invention was extraordinary, and how do you pick out truth from untruth, after all this time?" Mike told me.

'Stunned, sick and cold'

We know some truths because of Alison's private memoir.

It tells how she met the charismatic older man, a best-selling spy author, while working for MI6 in London in 1940, where he translated and she transcribed bugged telephone calls from London embassies.

She fell deeply in love, got pregnant, and they married.

But their life quickly spiralled into extreme poverty. He was first sacked, then imprisoned twice, and declared bankrupt.

"Stunned, sick and cold with terror", she was left desperate for somewhere to live with two lice-ridden young boys.

Even after his death, she was drawn into the deception, keeping her sons in the dark for decades about their father's other family.

Image copyright Wilson family Image caption Alexander and Alison Wilson met while working for MI6 in 1940

The other side of Alec's story has been pieced together by Prof Tim Crook, a journalist and academic who dug and scoured and prodded, tracking down the children from each of his four bigamous marriages, thereby helping to unite an extended family that was spread between Southampton and Edinburgh.

In Mrs Wilson, the battle for the truth is personified by Alec's two spy handlers - Coleman, the woman who ran the MI6 office in London, and Shahbaz Karim from Lahore - an invented character, since there are few records from his time in India in the 1920s.

Viewers are left trying to decipher how much of his story was devised by the secret services, just as we in the family are now.

According to Coleman's version, Alec was the author of his own downfall. His fall from grace was the rightful punishment due to a pathological pretender.

Karim, however, puts Alec's side of the story in Mrs Wilson.

His sacking and subsequent ostracisation by MI6, his convictions for wearing false uniform and embezzlement, were all part of the cover that would allow him to get close to fascists in prison and other intelligence targets.

This is what Alec told my grandmother in as many words.

This might seem far-fetched. And why would we believe a word Alec said when deception came so easily to him?

But we should remember that there are not just two possibilities - the hapless scapegoat, or the deluded fantasist - there is a whole host of possibilities in between.

'Most secret' letter

And Prof Crook has unearthed several anomalies which are hard to explain, and may lend credence to Alec's version.

Alec's dismissal from MI6 was apparently because he'd staged a fake burglary at his own flat. We know this because "C" himself (the chief of MI6) said so in a 1943 letter marked "most secret", which was released in 2013 following Prof Crook's first biography of Alexander Wilson.

But there are no records of any inquiry or prosecution for this fraud, and Alison makes no mention of it in her memoir.

While Alexander Wilson's output as an author is undisputed - he had more than 20 books published in the 1920s and 30s - his records at his main publisher have been lost.

Nor is there any trace of his membership of the Author's Club in Whitehall.

Could this be a "sanitisation" operation?

Image caption Fiona Shaw as Coleman, the woman who ran the MI6 office in London, in the drama

The biggest unanswered questions surround the claim that Alec had faked his translations at MI6, concocting reports implying the Egyptian ambassador was running a spy ring apparently to benefit Nazi Germany.

The spy ring was investigated by MI5, which eventually concluded (after Alec's sacking) that it did not exist and Alec's translations were fabricated.

But historical context suggests his reports may have been credible.

There is now plenty of evidence that some factions in Egypt were actively helping Nazi Germany during the El-Alamein campaign, on the promise of real Egyptian independence if Britain was defeated.

And Egypt's ambassador in London, Nachat Pasha, was a known nationalist activist who plotted to overthrow the pro-British government in Cairo.

MI5 files

What nobody disputes is that Alec was a fervent patriot.

He wrote many letters pleading to be allowed to serve in World War Two despite his injuries from the previous war.

Perhaps the spy writer had detected espionage in the Egyptian conversations, and was only embellishing his translations to raise the alarm?

Image copyright Tim Crook Image caption C wrote that Wilson had "a blameless record, first-rate linguistic abilities, remarkable gifts as a writer of fiction, and no sense of responsibility in using them!"

In the 1943 letters between C and other spymasters they agreed that Alec should never be allowed to work in an official capacity again. It is quite possible that they also warned publishers to steer clear of him.

His prolific writing career ground to a halt. That's why Alison - and her sons Nigel (my father) and Gordon - spent years destitute.

To make such a sweeping order the MI5 evidence against Alec must have been strong - but it is still being withheld.

It was not included in the partial release of files in 2013 for reasons of national security, leaving the family unable to form a rounded view.

Image caption The direct descendants of Alexander Wilson and their partners gathered in 2018. The four surviving sons Dennis, Gordon, Nigel and Douglas are seated in the middle

Personally, I'm dubious that the file would provide the exoneration that many of the Wilsons would welcome. But we'd still like the full picture.

"As things stand our father has been left in limbo as unreliable and a public danger," my father says.

"I think he was better than that, but if the files are released and prove otherwise, so be it.

"That would be a better conclusion than the current stench of concealment."

My dad is 74. Dennis, Alec's oldest surviving son, is 97. And Mike, the son from his second "marriage" who was told his father was killed in World War Two, died in 2010.

Prof Crook has appealed again for the report to be released, 75 years on.

As my dad says: "All we want is the truth."

Mrs Wilson is available to watch on the BBC iPlayer for the next 29 days (from date of publication)

Professor Tim Crook's biography of Alexander Wilson, The Secret Lives of a Secret Agent (Second Edition), is available via Kultura Press