Sitting talking to Sally Lane and John Letts at their dining table in their terraced house in Oxford, a series of questions nag in my head. One is: “What might be the limits to parental love?” As husband and wife piece together once again the story of how their eldest son, Jack, then 18, took himself off to Jordan on holiday five years ago and ended up in the newly formed Islamic State in Iraq and Syria – a journey that eventually landed him in a Kurdish prison camp and put them through a hellish court ordeal – it is hard not to wonder just a little about that word “unconditional”.

Their recent court case at the Old Bailey, three and a half years after their initial arrests, itself seemed designed to test that idea to breaking point. Letts, 58, an organic farmer, and Lane, 57, who had worked in publishing and for the NHS, were convicted on one charge of “funding terrorism”, and acquitted of another. A third charge was left on file after the jury failed to reach a verdict. Their crime had been to send, at the request of their son, the sum of £223 to an intermediary in Lebanon in September 2015, in part to pay for a pair of glasses. There was no suggestion that they believed they were sending the money to Isis, the court accepted, but it found them guilty of knowing there was a risk that the cash might fall into the wrong hands.

They were found not guilty of trying and failing to wire a further payment of £1,000 later that year. That was needed, they believed, for Jack to pay a people-smuggler to help him escape the “caliphate” after he had denounced Isis as “un-Islamic” and had apparently been on the run in fear of his life.

It’s nice that we are not in prison, I suppose. Though at least if you are in prison, you know there is nothing you can do John Letts

Letts and Lane were sentenced to 15 months in prison, suspended for a year. The investigation and legal case cost an estimated £7m. The verdict and sentence were, in Lane’s view, engineered so “everyone went home happy. All that money did not appear to have been wasted. We didn’t go to prison. The prosecutor was beaming at the verdict, because she had made her point. And the jury, who by then appeared bored silly by the whole thing, could finally go home.”

When I sat down with them a week after this verdict, last Sunday afternoon, I began to ask if they were relieved the trial was over, but their tense and exhausted faces made it clear such a question was beside the point. One long legal case has simply given way to another: the battle to get their son, now 23, home to face justice here – or in Letts’s native Canada – from the north Syrian prison camp where he has been held for more than two years without trial. “There is no less stress now,” Letts says. “It’s nice that we are not in prison, I suppose. Though at least if you are in prison, you know there is nothing you can do.” Instead, their waking hours are consumed with framing arguments to governments who have no wish to hear them.

I first met Lane and Letts back in January. I had come across a Twitter account that Lane had set up in which she mentioned how the local jobcentre had advised her to move cities if she wanted to find work. I’d contacted her in the first instance to sympathise with that situation and to see if she might want to talk about it. Over a lunch of home-made soup and bread she and her husband went through the nightmare of the previous years, in which they had not only lived in constant fear for their son, but had also lost their livelihoods and something close to their sanity. A contempt-of-court restriction had left them unable to speak in public about any aspect of their situation or Jack’s.

After lunch that day John Letts, who has a contrarian’s intellectual energy, took me on a drive to introduce me to his former life. He had worked for a time as a roof thatcher when he first came to Britain in the 1980s, and had subsequently revolutionised the idea of “heritage grain” with 15 years of research into ancient strains of wheat and rye, culled from old thatch, to redevelop lost varieties with unique sustainable qualities. Prince Charles was a fan, and grew his crops at Highgrove; to celebrate the partnership Letts had given the prince a scythe as a present, with a Canadian maple handle.

The photograph of Jack Letts in Syria that accompanied the first news stories about him. Photograph: Counter Terrorism Policing South East/PA

At about the time Jack went to the Middle East Letts had finally got the business to the point where it was gaining national and international recognition. Because of restrictions placed on his and Lane’s time and finances since their arrest, however, he feared his research would be lost and passed his work on to others. He took me to a few places where his knowledge still thrived: first to Hamblin’s bakery in Oxford, which used his magical wheat in its fabulous bread, and then to Toad, an artisan distillery that was using the ancient rye grains he had developed to make unique gins and vodkas.

While we drove Letts had elaborated on the ways in which he had lost his son. He stressed some of the difficulties Jack had faced, mental health issues that saw him drop out of the sixth form of Cherwell school nearby, and the problems of getting help for him. Jack had been diagnosed with an obsessive-compulsive disorder and with Tourette syndrome. Letts painted a picture of a troubled but highly sociable teenager, a boy who had sailed through his GCSEs, who had previously been identified as gifted in English and maths; a son with whom he loved to argue the toss about religion and politics and everything else.

When Lane and Letts appeared in court a very different narrative was told about Jack. The prosecution focused on the messages that had been uncovered from online accounts in his name. The most infamous of them included a savage death threat, a wish to perform “a martyrdom operation” against an old school acquaintance who had joined the British army. Sally Lane had confronted her son about that post and threatened to disown him, but now both parents maintained that his accounts had been hijacked by Isis sympathisers. Lane was cross-examined for three days on every exchange she’d had with her son. Letts did not give evidence on advice from their lawyers. Instead the prosecution went line-by-line through his emails to Jack. “A father should never live to see his son buried,” Letts pleaded to him in September 2014. Another message read: “Jack, I’ve never begged you before but I am now. Please contact us. Your mother is collapsing with fear and sadness … You don’t have to die to help your fellow Muslims.”

The five long years of worry have taken their toll on them as a couple. They are so familiar with the complex nuances of the case that it has become a kind of tetchy shorthand between them. When I arrived at their home last week I was not out of the hall before they were in a clipped argument about what documents they could send me to shed light on Jack’s current situation. At the heart of all this remains their attempt to square the child they raised – the child present in golden, smiley photographs of family holidays – with the figure conjured by the prosecution in court. They trace this disjunction back to the Sunday Times article of January 2016, which coined the name “Jihadi Jack” for their son alongside a now familiar picture of him in combat trousers in Raqqa. The story gave Isis a white and middle-class face for the first time – stoking a fear in every parent with a child on a gap year.

John Letts and Sally Lane outside the Old Bailey in London after their conviction last month. Photograph: Yui Mok/PA

In the public mind it is that image of “Jihadi Jack” that has taken hold. I have listened to enough radio phone-ins and followed enough social media to know that the loudest public voices agree with the TV verdict of Piers Morgan on the case, that he didn’t care if Jack Letts was left to rot in a Kurdish jail for ever: “If you go to fight in Syria, that’s the way the cookie crumbles.”

That is not what Letts and Lane can allow themselves to think. They point to the fact that 400 of the 900 British nationals who went to Syria to support Isis have now been repatriated. (The home secretary, Sajid Javid, gave a statement confirming that number in February, in a Commons exchange in which it emerged that “only about 40” of those 400 had or would face trial.) You can see how, as this quiet “policy” has been enacted, it might have been useful politics to keep one or two vivid individuals still in Syria in the public eye. Shamima Begum, the Bethnal Green teenager, was the first. Since she has been stripped of her British nationality and after her child died in a refugee camp, the spotlight has shifted to Jack Letts.

Jack’s case for repatriation has not been advanced in recent months by the attention surrounding his parents’ trial. A series of TV interviews with him from prison included his admission to the BBC that he had been “an enemy of Britain” and once would have been prepared to carry out a suicide bombing – though he expressed what looked like heartfelt regret for those positions now.

I have always said that if we find something he has done wrong, I will be the first to condemn it.

His parents offer two determined arguments in his defence. The first is their belief that when he gave those interviews he was under duress from his jailers. They have little detail of the conditions in which Jack has been held, beyond an early report he gave to the Canadian authorities in which he said he was in a cell of 30 men and only eight beds, and they were allowed out for 20 minutes a day. He maintains that he has been tortured in custody.

That view is supported by the civil rights advocate Clive Stafford Smith, the founder of Reprieve, who met Jack in prison. He spoke to him a couple of weeks after the BBC report and said that Jack wished to make clear the circumstances of that interview: “He had been taken to a place proximate to where he had been tortured, had been asked many of the same questions that he has been asked under torture, and he had been bound to give the same false answers because the people in charge of his torture were there.” The BBC contends that Jack Letts gave full consent to the interview and thus all protocols were followed.

Jack on his father’s farm near Oxford in 2006. Photograph: Courtesy of Sally Lane & John Letts/Courtesy of John Letts

Jack’s parents’ second argument in mitigation is that their son went to Iraq for humanitarian reasons; however sympathetic he might have been originally to the idea of the “caliphate”, he was a gentle soul, never a fighter. “He was a white Englishman, surely Isis would have used him in recruitment videos and so on if he was involved…” Lane says. “Or there would be pictures of him posing with guns.”

“He was always talking about learning Arabic so he could help in a hospital or teach,” John Letts says. “He was inspired by the Arab spring and horrified by the war in Syria and the 6 million refugees.” John clings to a last exchange of an ITV interview where his son said of his prison, “If I have to stay here longer so you can get the women and children out first, that’s OK”.

“I was really proud of him saying that,” Letts says. “Of course people will read that and say: ‘You are proud of Jihadi Jack – you have to be insane!’ But I’m sorry, from my perspective he is still a decent kid. I have always said that if we find something he has done wrong, I will be the first to condemn it. Until then, am I supposed to just believe what the Daily Mail says about my son?”

Lane subsequently sends me the printouts of messaging exchanges they had with Jack while he claimed to be in hiding from Isis in Raqqa between March 2016 and July 2017. Initially he was trying to organise for them to send money so he could escape. Some of the exchanges between mother and son capture the cadences of family banter; others take in the surreal contrasts between Oxford and Raqqa, food shopping and Russian air strikes. Often Jack responds to questions about his safety, about his lack of money, with a belief that Allah will provide. In order to be sure it was Jack talking, Lane occasionally asked him little bits about his past. In one of the last messages, after the fall of Raqqa, there is the following exchange:

Sally: I’ve been so worried. OK, I’ll test you. What was the name of the head of discipline at your school who gave u C3s? Are u OK?

Jack: I’m fine, Mr Davis?

Sally: Yes

Jack: So many people used to give me c3s. What’s new? I’m out by the way.

Sally: We’ve been worried sick. Out of the country?????

Jack: Out of what they call Isis territories.

Jack Letts is interviewed on ITV News in February. Photograph: ITV/PA

In them, too, in between doctrinal religious quotation, you get a little sense of Jack’s quick wit – if things had taken just a slightly different turn, Sally tells me at one point, she’d imagined Jack would have not been in a war in Syria but on the stage at the Edinburgh fringe with his friends.

One of the unspoken subtexts of the court case was the way Jack’s conversion to Islam, when he was 16, seemed to exacerbate his OCD. Did they recognise that link?

“He had cognitive behavioural therapy for the OCD when he was younger,” Lane says, “and that seemed to work. But it came back again after he converted. One doctor actually told us that the religion was a useful vehicle for his OCD. With all the praying and washing.”

“Some Muslims Jack knew had told him that [the OCD] was the evil spirits of the djinn that had been sent to test him – a load of crap – and that the solution was to wash more, pray more,” John says. “He was getting up at two o’clock in the morning and praying, and washing, so he never slept more than a couple of hours. He started to fall asleep in class – A-levels became impossible.”

Alongside those habits they concede that Jack developed an obsessive “search for the truth”. He read everything he could find about Islam and about what a perfect Muslim was. “He had this massive grammar book of Arabic that he would carry everywhere with him,” Lane says. “He took it to America on holiday with him.”

“All OCD is about fear, if you don’t go through the rituals then something bad will happen,” Letts says. “In Islam that becomes, if you don’t [live according to the religion] you are going to burn in hell. He was obsessed with not doing something wrong. That is why he wanted to learn Arabic so he could be the best Muslim he could be.”

It is also, it appears, one reason why he ended up in Islamic State, to test those abstract notions against reality. His parents want to believe he was quite quickly disillusioned. They admit that they do not know what Jack was doing in Syria for several months. But when he reconnected with them in early 2016 he was, according to their reading of his often cryptic messages, holed up in friends’ houses in Raqqa, dodging Russian bombardment, nursing a shrapnel wound.

Their faith in him still being who he was is what sustains them, really. Is it possible for them, I ask, to entertain the idea that he had been brainwashed, targeted by people who might seek to radicalise him?

“What you are saying is: are we in denial about him?” Letts says. “That is what everyone is throwing at us…”

The Letts family on holiday in Snowdonia, with Jack, aged 12, on the right, and his younger brother Tyler on the left. Photograph: Courtesy of Sally Lane & John Letts/Courtesy of John Letts

I say I can only imagine the difficulty of wrestling with the idea of the boy they loved so much and this different man who is being presented. In January 2015, Jack got married to the daughter of an Iraqi tribal leader and they subsequently had a baby. Was that in character for the boy they knew?

“I wasn’t surprised he got married,” Letts says. “Though I was disappointed not to have been at his wedding. He shows me a message in Arabic Jack tried to send to his wife through the Red Cross, a love letter: ‘I tried to get to you and little Mo in Mosul three times and I couldn’t get through – Isis were going to kill me. I will never stop trying to find you even if it takes 40 years.’”

Lane says she thinks of the apparent ease of Jack’s integration into Sunni Iraqi society in terms of a family holiday they had in Morocco when he was 17. “Jack just had dinner with a different set of people each night. He’d get talking to people and they would invite him back to their house. That is the way he was.”

The prosecution took close account of one email that John sent to a friend, soon after they first heard the devastating news that Jack was in Syria. “I’ve lost him,” Letts wrote. “I feel terrified, betrayed by Jack, embarrassed, massively let down. I know he’s not a nasty boy. But he’s very stupid and I created him, with my armchair revolutionary shite.”

What did he mean by that?

“I was in shock and fearing the worst,” Letts says, “and feeling guilty that I had raised him to support the underdog.” He points to the fact that in the past he had been involved in campaigning about East Timor, for example, to the extent that they had Timorese activists living in the spare room. “I wouldn’t call myself a radical lefty as some people are doing. But certainly Jack was raised to question oppression and imperialism. We took him on the Iraq war march and all this kind of stuff. And at that moment [when I first heard he was in Syria] I was thinking, ‘Am I reaping what I sowed?’ That’s all.”

If they are guilty of anything, it seems to me, it is of a style of parenting determined to respect above all the individuality of their teenage son, his right to be intellectually curious, to pursue “grand adventures”. Hindsight is a wonderful thing, but is that something they recognise?

John Letts and Sally Lane demonstrate outside St Paul’s, October 2017. Photograph: Philip Toscano/PA

I have two regrets,” Letts says. “One is that when he converted to Islam I didn’t walk with him on that path, by being more attentive to what he was doing. But, you know, my business was going under and I was trying to farm and for a time I maybe didn’t give him the time I should have. He saw me probably as this atheist dad who had arguments with him and I maybe should have backed up and said, you know, just ‘that’s interesting, son’. And the second is that I think now his OCD was worse than we ever addressed. It was hard to get help for him when he was a child, and once he had become an adult there was little we could do. Those are my two regrets. But I honestly can’t say I was worried when he left…”

“You have to remember,” Lane says, “he was going to Jordan with his mates…”

While the legal battles have been front and centre, Lane and Letts’s lives have fractured in other ways. A few good friends have stood by them but inevitable strains show between them as a couple. Their younger son, Tyler, 21, has been away at university for much of the ordeal, though his AS-level revision was thrown off course when the police seized a computer containing all his notes. The time consumed by the legal battle has left Letts and Lane in “a deep financial hole”. After Letts shared the painstaking results of his research, the movement he pioneered in heritage grains has picked up momentum without him. Lane has had to move from job to job, having been asked to leave several when colleagues discovered she was a “terrorist suspect”. On the morning that their trial began, they received a bailiff’s letter to say their remaining possessions would be taken away.

It seems a bit hopeless to ask them about hope, but I wonder if they can, with the trial over, think of getting their lives back on track?

“Not until Jack is safe,” Lane says. “I hope that is not long. There was an Italian handed over to the Italian government last week…” As a mother she says there’s nothing she can enjoy until then; the baking summer afternoon outside is just a reminder of the heat in her son’s prison.

“Jack has become the most politicised figure of any of the detainees,” Letts says. “Obviously [the authorities] would probably most like him to ‘fall down the stairs’. But we are not going to go away. I am hoping if we raise enough pressure, he becomes such a pain in the ass for everyone that they will hand him over too.”

They are, despite everything, Letts suggests, placing their faith in British – or Canadian – values: “The rule of law, innocent until proven guilty, the rights of the individual…” That, and their stubborn love for their son.