Richard Ratcliffe is recalling the moment, last Sunday, he discovered his wife Nazanin could face an additional 16 years in an Iranian prison on fresh charges of attempting to overthrow the Islamic Republic.

“I was really shocked. Different Iranian officials had been signalling she’d be eligible for early release next month.” Sometimes, he says, it’s hard to understand what’s going on. Then he corrects himself: “Actually, it’s always hard to understand what’s going on. But everything that’s happened has clearly had a political timing to it.”

Nazanin Zaghari-Ratcliffe, a British-Iranian charity worker, was about to return to the UK from a holiday visiting her parents with her 22-month-old daughter Gabriella when she was arrested 18 months ago – shortly after economic sanctions on Iran were lifted and its government swapped prisoners with the US as part of a landmark nuclear deal. Along with academics and international workers from other western countries who were detained around the same time, she was accused of espionage by members of Iran’s Revolutionary Guard, then separated from her daughter, placed in solitary confinement and sentenced to five years in prison.

Gabriella, who has sole British citizenship, was prevented from returning to her father in the UK and placed in the care of her Iranian family, her passport confiscated. “She would wake up in the night, crying,” Ratcliffe says. “She kept going to the door and pointing at photos of us, asking her grandparents for her mummy and daddy – particularly her mummy. The first visit she had with Nazanin in prison, she didn’t say anything. She just sat on Nazanin’s lap, stroking her face for 15 minutes and looking at her, not quite believing she was there.”

An appeal to overturn 38-year-old Nazanin’s sentence was refused in January, and Ratcliffe has been denied a visa to visit his wife and daughter: “The basic cruelty of splitting up a family is quite breathtaking.”

It feels like the Iranian government is torturing his family, he says. “Nazanin was promised she would be released several times and each time it was taken away. At the beginning I thought: is it incoherent? Is it just someone promising sincerely and then being overruled? Now, I think: no, it’s a casual cruelty – and it’s systemic. There’s a deliberate process of trying to pacify the families of political prisoners, to keep them hopeful so they don’t do anything.”

Iranian officials have complained to Foreign Office officials – who meet Ratcliffe every three weeks – that his behaviour is unreasonable, and the British government has advised him to scale back his campaigning in the public eye, he says. But he’s not going to stop. “I’m really clear: I’m going to keep campaigning until they’re home. And I’m going to get louder and louder. I’m going to step up and step up.” His message to the Iranian government is: “If you create these crazy new charges, I’m going to tell you they’re crazy. If you want this to end, get them home.”

He draws his strength from the messages of support he receives from members of the public who join his campaign to free Nazanin. “Lots of people, old and young, will post lovely comments, and that really sustains me. Most are just ordinary people, sometimes not with anything to say except: we care.” His Change.org petition has attracted almost a million signatures. “That genuine reaching out that people do, that kindness, is really nurturing. It keeps me going.”

He plans to show all the messages to Nazanin one day. “It’s almost like a photo album of other people caring, and part of my campaigning is about her having something to come back to. It’s making it clear to the outside world that she’s innocent, that this is a mockery – it’s political and it’s got nothing to do with anything she’s done.”

When he’s not busy campaigning, Ratcliffe, 42, is trying to hold down a job as an accountant, and so we meet at lunchtime at the Barbican, near his central London office. He should blend in with other City workers in grey suits, but the strain he has been suffering is palpable. “I look a fair bit older than I did last spring. I completely lost the ability to sleep and to work at first.”

There have been a few moments when he’s allowed himself to cry – such as when he discovered Nazanin’s appeal had been rejected – but most of the time he tries to stay in “campaign mode” and focus on the future, with varying degrees of success. “It’s in the quiet moments that the feelings rise up and wash over you. There is a way in which it’s still easier for me to battle on and not really unpick all the feelings, because I’m not sure that’s really helpful.”

It is the separation from Gabriella that he finds the hardest. “A baby went out there and a little girl will come back. That bit in the middle is gone for ever.” It’s particularly painful, he says, that Gabriella has lost her ability to speak English and now only speaks Farsi, which he doesn’t understand. “Emotionally, what Daddy meant before and what Daddy means now are two different things. Now, Daddy is just the guy who speaks to her in English on the phone.”

The one phrase she can still say in English is “I love you”. “She makes a heart with her hands when she says it.” Big enough now to articulate that she misses him, she asks: when am I going to see you? “But a lot of the ways you would reassure a three-year-old, I can’t do. There will need to be a long process once she’s back to make sure she knows, deep down, her parents didn’t abandon her.”

Twice a week, Gabriella is now allowed to have a 45-minute visit with her mother, who was only taken out of solitary confinement at the beginning of this year and has since been diagnosed with advanced depression. “At one point, Nazanin was suicidal. She felt it would be better for everyone if she just killed herself.” The debilitating psychological effects of her imprisonment have had physical consequences. “There was a period when she was having regular panic attacks, couldn’t walk, couldn’t lift things up. Her hair was falling out for a long time. She regularly loses her appetite.”

She tries to cope, Ratcliffe says, by remembering her life in the UK. “It’s part of keeping herself mentally active. Even things like: can she remember what’s on the kitchen work surface, what’s near the pots? She always says on the phone: please do whatever you can to get me home.”

The visits from Gabriella are her lifeline, and he has promised he will never take their daughter back to Britain without her consent. “It was really important to promise that to her when she was in solitary confinement.” But the return of Gabriella’s passport in June gave them hope that their cause was moving forward. “I was feeling like we were at the beginning of the end – so last week’s news was a big surprise.”

He has taken hope from a statement the Iranian embassy in London put out to the Iranian press, saying there might be a mistake about the length of Nazanin’s new sentence because the prisoner’s husband “often gets things confused”. But he believes the Revolutionary Guard is using Nazanin as a pawn to embarrass the Iranian and British governments and will not give her up unless it gets something in return.

Despite his best efforts, there are moments when the situation gets to him. “Being around the flat is the place where obviously they’re not. And it catches up with me when I look through old photos.” Sometimes, when he wants to feel close to Gabriella, he visits the slide in the park she used to play on, near their flat in north London. “I’m a middle-aged man, with no reason now to be wandering around a playground, but I’ve been there a few times, just to remember her solemnly going up and down the slide again and again.”

He tries, as much as he can, to focus on the future, imagining what they will do together when they are all reunited. “I’m looking forward to the Saturdays when we’d all go off and do something, go to the park, feed the ducks, or take a day trip to the beach. That’s what’s in my head, that’s what’s waiting.” He hesitates, and for the first time his voice wobbles. “Yeah. And I’d probably just hold them,” he whispers.

Even now, he won’t allow himself to believe Nazanin won’t be released soon. “There’s no reason it couldn’t all be over, because it is all nonsense. So what I’ve experienced is: she’s not going to be home for Christmas ... or she’s much less likely to be home for Christmas ... but I’m hesitating to even articulate that. I won’t accept it. I have faith that in the end, sunlight clears injustice. And I’ll carry on until it does.”