In March, as a viral video highlighted the atrocities carried out by Ugandan warlord Joseph Kony, 24-year-old Yorkshireman David Simpson stumbled across the mutilated bodies of 13 men in an African forest. He is now in jail charged with their murder

Given that he's in prison in the Central African Republic charged with the massacre of 13 people, it's really not as hard getting hold of David Simpson as you'd think. I just ring him up. And there he is, on a surprisingly not-too-bad line, sounding a bit relieved to have someone to talk to.

What's more difficult, even after three hours on the phone, is making the mental leap; understanding what it's like being imprisoned in an African jail, in a country where there's no due process, accused of a crime that carries the death penalty. David, 24, is from Yorkshire, and like his family, whom I'd met a few days previously, he's not given to spontaneous outbursts of emotion.

Prison is now "not too bad" he says. It's better than when he was first arrested, when he was sharing a cell with 37 other men "and we were so tightly packed in, you had to sleep on your side spooning one man in front of you and one behind". When "there wasn't even space to lie on your back" and they were locked up for 23 hours a day.

He's now in a cell with 12 others. He's still got a mobile phone. He had two before, both stolen. And he's hopeful. Even in the absence of any concrete news, he's hanging on to the knowledge that he didn't actually kill 13 people in a ritualistic fashion and that, eventually, somebody, somewhere – the British government? – might actually try to get him out. It's hard to share his optimism on this last count: if the Foreign Office really is working hard behind the scenes to do something, they're doing that something very, very quietly indeed.

David is at the mercy of fortune in a country with a barely functioning government and almost no rule of law. Officially, you can question suspects for 72 hours before charging them: David was held for six weeks. Still, he says, it could be worse: "When we were admitted to the prison, we were summoned to the commander's office so he could tell us what bad people we were, and there was a board which stated there are 668 prisoners in here, but only 77 have been convicted of anything. And some of them have been here five years or more."

It's in David's character, it turns out, to say that things could be worse, though by most people's reckoning they're already pretty bad. Three months ago, in a remote corner of the Central African Republic, where he was working as the general manager of a safari company, he walked into the forest and unwittingly uncovered a massacre: the bodies of 13 badly beaten and tortured men.

"Two of our employees had found the bodies and I got a call from my boss saying get down there. It was down by a river, where men from the nearest town, Bakouma, come to mine gold. It's illegal, and we'd been down there the month before, to tell them to leave. This time, as we approached, we saw six bodies. They were tied together in a circle, face down, and had been beaten to death with sticks. Some of them were naked or partially naked. It was so brutal. A couple of them, their faces were completely smashed up. Then methodically every one of them had a machete cut in the back of the head."

He called the authorities and the next day returned with the military and uncovered another seven bodies. "That was a lot worse. Some of them had been tortured before they'd died. They'd had boiling water poured over them. It was pretty bad. And the first day there was no smell but the next day, the smell … it was in a valley and the military guys were throwing up."

The investigation took 30 minutes. "They took some photos of the bodies on their mobile phones, including these sort of trophy ones, where they stood over them with their guns and their sunglasses on. And then the commander said it was too dangerous to stay, so everybody left and that was it. That's all the investigation there's been. No one has been there since. There probably won't even be many bones left now."

The evidence suggests that this was the work of the Lord's Resistance Army, a militant group of forced child and ex-child soldiers led by the Ugandan warlord, Joseph Kony: the way the bodies had been arranged, the manner in which they'd been killed, previous attacks they'd carried out in the area. "We took one of our trackers and he found footprints, wellington boot prints, which are standard LRA wear. And it just has all the classic hallmarks of one of their attacks. The way they take clothing. And use sticks to beat them. And we've had other attacks in the area in the past year."

And yet a few days later, Erik, David's Swedish boss, was arrested in the CAR capital, Bangui. Ten days later, David was too, along with 11 other employees. "There was another group of men nearby when we discovered the bodies. One of them panicked and ran away, and he started the rumour that we'd killed them. They thought we'd done witchcraft. There was an official claim in the newspapers that I gave them food with magic powder in it, and then I beat them to death with a stick."

Gillamoor, a pretty village folded into the rolling hills of the North York Moors, is not the sort of place where you'd expect to confront the brutal realities of child soldiers, torture and rebel forces.

It's certainly not how David's parents, Peter and Vicky Simpson, thought they'd be spending this summer. Not at the height of the pheasant-rearing season, when their game farm occupies them 16 hours a day, seven days a week. They've 75,000 pheasant chicks to feed and water and send off to their buyers. And when they and their younger son, 22-year-old Paul, tell the story in the kitchen of their house, their four black labs playing at their feet, there is a lot of what psychologists like to call cognitive dissonance at work, or what the rest of us might call an almost total lack of reality. The conversation lurches between a discussion of the vagaries of pheasant breeding, to Joseph Kony and the Lord's Resistance Army.

Back in March, the Simpsons had barely heard of Kony. And yet, just at the moment that they were learning his name and methods, so, in a bizarre coincidence, was the rest of the world. Because, back in March, you couldn't avoid Joseph Kony, even if your son wasn't in jail accused of carrying out a massacre that Kony's forces are likely to have committed.

After nearly two-and-a-half decades of terrorising northern Uganda, and later the Democratic Republic of the Congo, South Sudan and the Central African Republic, abducting and brutalising children and killing their parents, he had suddenly become news. Invisible Children, a Californian charity and advocacy organisation, made a film about Kony. In fact, it was another film about Kony – they'd already made more than 150 films about him, some of which had been seen a few thousand times. But Kony 2012 would turn out to be different.

Almost immediately, links appeared on Facebook and Twitter, alongside Beyoncé videos and news stories about Kim Kardashian. Within days, Twitter's top 10 worldwide trending topics started to include a whole range of new words: StopKony. Kony2012. Uganda. Invisible Children.

Within 72 hours, 43 million people had watched it. Within six days, 100 million. Somehow, Joseph Kony had become the subject of the most viral video of all time.

But almost immediately there was a backlash, with critics accusing Invisible Children of everything from lacking transparency to having "white saviour complex". David, however, can't help but see his fate wrapped up in Kony 2012's fate. "I think my story has only really been in the paper because of it. If it hadn't been for that, would anyone have been interested in me? I think if I was just some guy who found some bodies … well, it makes it a little bit more of a story, doesn't it?"

It's hard to disagree. In the film, directorJason Russell states that the aim of Invisible Children, an organisation he and his friends set up after encountering Kony's child victims in Uganda, is "to make Kony famous". And, in the reams of debate and counter-debate, nobody's denying that he now is.

It's just a peculiar set of circumstances that has brought together an old-fashioned story of a young man in search of adventure who comes unstuck in darkest Africa, and the latest new digital phenomenon. But then the video itself demonstrates the clash of two cultures: on the one hand, there are the complex, brutal realities of central African politics; on the other, America's view of it.

And in the middle of it all is David. Or "the beast of Bangui", as his father has taken to calling him. They take no prisoners in Yorkshire. If it's not David they're lampooning, it's each other.

"You can't be emotional on the phone with David," says Paul. "You've got to just treat him like you would at home and make fun of him." And then he adds pointedly, motioning to his father, who had come a little undone a month or so previously on the BBC: "Unlike some people who are just soft and go on national telly and cry."

"The room was hot," says Peter. "It was sweat."

All David wanted was what thousands of young men have wanted before him: an adventure. Until two years ago, he worked as a manager in a factory in Sheffield. "For my age I was doing pretty well, it paid quite well, and I would have probably moved up the corporate ladder. But I didn't like it, it wasn't me."

He'd always wanted to fly and for his 18th birthday his parents bought him a flying lesson. "And that was it. It wasn't like they bought me a whole set, they bought me one lesson, but I was hooked. It took me three years to save the money to take the rest. But I did, and I got my pilot's licence and that's what I wanted to do: I wanted to travel. I would have gone anywhere."

He sent off, he estimates, 300 emails. "And I got three replies. One saying, 'No thanks'. One saying, 'Maybe'. And one from a man who ran an African forum who said, 'put a post up and maybe someone will read it'. And someone did – Erik."

Erik is Erik Mararv, a Swedish national who has lived all his life in the Central African Republic, and who owns a company called Central African Wildlife Adventures (Cawa Safaris). "I had no experience but somehow Erik saw something in me and offered me a job."

What do you think he saw? I ask.

"I don't know," he says though he offers something of a clue. "I told him money's not important. I said I was willing to work for free to prove myself. He flew over and came to meet me and he said he'd never seen a family that worked as hard as mine. He was really impressed by my mum. Couldn't believe how hard she works. And he took me on, covered all my costs and so on. And at the end of the year, he said, you're the best guy I've got and I'd like you to be my general manager."

The eastern part of the Central African Republic is one of the last true wildernesses on Earth. There are barely any roads, let alone towns. "It really is an incredible place. There are places I've been where nobody has ever been before. No one." David loved central Africa, and the challenge of his job. He'd learned Sango, the local language, and was managing 250 employees, but he was entirely unprepared for what happened next.

"We thought we'd just be held and then they'd release us. The minister of justice said if we paid 20m CFA francs (£24,000) we could go free. That was the official price they gave us. We were thinking, do we pay this? We didn't have that money, we didn't have that cash, we would have to sell assets to be able to pay that. And then we became serious suspects."

Human Rights Watch happened to have someone in the area at the time documenting LRA violence and Anneke Van Woudenberg, a senior researcher in its Africa division, said that there was very strong evidence not just that it was the LRA who'd carried out the attacks, but that information had clearly been manipulated by the authorities for their own purposes.

"We didn't set out to get involved but all the early indicators were that it was LRA so we sent our researcher to Bakouma. And it was from there that we discovered, not just all these classic hallmarks that it was an LRA attack, but that there was also strong evidence of manipulation of the information.

"There had been some friction and tension between the miners and the safari company. And then one false witness started a rumour and the authorities built a narrative around it seemingly to suit certain people's interests."

The one false witness she refers to was the man who ran away from the scene. The narrative was the accusation that "the white men" had initiated the massacre. And the "interests", David claims, are a network of people who stand to profit from their conviction.

Human Rights Watch wrote a public letter to the minister of justice, but, apart from that, there's been little help from any quarter. Erik Mararv's father, a well-connected Swede who used to own the biggest well-drilling operation in the CAR, has contacts and influence. But even this hasn't helped (though Erik has managed to leave prison temporarily on "sick leave").

Back in Yorkshire, David's father and brother tell me about the trip down to London to the Foreign Office. "They just said to us, 'We have 20,000 British people locked up abroad, most of whom are in a worse position than your son.' "

"And Anne McIntosh, the local MP, she gave us a right bollocking," Paul reminds him. "Oh yes, it was like going to see the headmistress," says Peter. "She said, 'First of all, David has gone to a country that if you type it into the Foreign Office website, it flashes up in red saying don't go there. And if you type the area of the country that he was working in, it flashes up saying definitely don't go there. And now you're asking for taxpayers' money to spend on getting him out?' We kind of just sat and looked at each other and then she said, 'Oh, you think you are big and clever with all this media but you are doing no good whatsoever.'"

But what else is there to do? It doesn't help that the Central African Republic doesn't have any diplomatic representation in Britain, or vice versa, but given the seriousness of the crime, the lack of any evidence, and the failure to follow any proper judicial process, the official UK government response has been underwhelming to say the least. You do have to wonder who you have to be connected to, what friends in high places you need to have, for there to be any political will to help at all. In this respect, Britain and sub-Saharan Africa are perhaps not so different.

It doesn't help either that Cawa Safari was in the business of taking rich men trophy hunting. The concession covered two-and-a-half million acres, or "two Yorkshires" as David's father describes it, and his clients came from America and Europe on what were billed as the ultimate hunting experiences: they paid up to $20,000 (£13,000) to shoot a single animal, many of them spending $100,000 or more on a single trip.

"Many of them are here for the experience, it's not just about killing things. Some of them are, they want the trophy on the wall. Though I don't understand that myself," says David. "But, we might spend a week tracking an animal. We only shoot old males. We have strict quotas. And if we weren't there, the poachers would be. All our trackers are old elephant poachers. We've 25 of them who'd shoot up to 10 elephants a year. That's 250 elephants that are not being shot.

"If we weren't there, the animals would disappear, like they already have in the north."

It's brought hate mail to the family though. And threats and abuse. "My mother got a letter saying I should be raped and killed for what I do. It's been harder for my family than it has for me in some ways," says David. "I can just get on with it. And I have a better understanding, I've lived here. Dad has hardly been out of Yorkshire. I feel bad for what I've put them through. I really think it's been easier for me. I've got a six-pack for the first time in my life. And I've got a mosquito net. I didn't have one before, which is why I kept getting malaria."

He's relentlessly positive. Most of the time. "I exercise. I've had a good supply of books. But sometimes … I get so angry I just want to hit something. I know that it does me no good. It doesn't help so I calm myself down. But I'm just locked up. They've taken three months from me. How much more will it be? And they know I'm innocent. Even the judge has said that. There's no logic in the case they've built, no evidence."

Three months after the arrests, David is still in prison and Kony's forces are still at large. But in that time a lot else about central Africa has changed.

"I've been working on central African issues for 13 years," Anneke Van Woudenberg from Human Rights Watch tells me. "I've been documenting the LRA since 2006 and Human Rights Watch has been doing so since the late 90s. And we've never seen anything like the kind of interest that Kony 2012 created.

"It was astounding. And incredibly exciting. Because whatever one thought of the video itself, it massively raised awareness of Kony. A lot of people were talking about it. And awareness is the first step in pushing for policy change. And we have seen real policy changes, especially from the UN."

It's unlikely this will help David though. Internet trends may move fast – #kony2012 disappeared almost as meteorically as it arrived – but the wheels of central African justice move altogether more slowly. Though not as slowly as the Foreign Office.

It's quite hard hanging up the phone, finally, on David. He sounds very alone. And very far away. I'll ring you back, I say, if I can think of any other questions. "I'm not going anywhere," he says. And that's the worry: that he's not.