Niall Dunn (pictured) was the non-identical twin brother of Harry who was killed by the fugitive wife of US spy in August

When Niall Dunn remembers his twin brother Harry, there are no shortage of happy memories which come to mind.

Their shared love of SpongeBob SquarePants and Star Wars, for example, or their coveted matching Thomas the Tank Engine pyjamas.

The time that Harry wore a sombrero during the entire flight home from a family holiday to Mexico because he couldn't bear to put it in his luggage.

Every night in their shared bedroom in the peaceful village of Charlton, Northamptonshire, they'd stifled giggles as they played guessing games and whispered song lyrics to each other in the dark.

They fought, like any brothers, but their bond as non-identical twins was nevertheless so strong that they had an instinctive understanding of each other's moods.

Entire conversations could be held in a single glance. But today, as Niall recalls that 'very special' childhood, there is one memory in particular which refuses to budge.

Late one night, when they were both aged six, Harry saved Niall's life. He had woken struggling for breath, in the throes of an asthma attack. Harry – the older twin by two minutes – had understood the danger, and rushed to wake their mother, who called an ambulance.

At the time, it solidified their bond. But what haunts Niall now is the terrible knowledge he was not able to save his brother in return. 'I've returned to that memory a lot over the last few weeks,' Niall says, his face crumpling.

Niall says that, until now, he felt unable to join in his family's campaign for justice as he is naturally reticent and socially awkward - the opposite of his extrovert twin brother. Pictured: Harry (left) and Niall (right) on their first day of school aged five

'He saved my life – I owed him one. He was always so good to me, so protective. But I never got to pay him back.

'These are thoughts I haven't yet been able to process properly. I'm not sure I ever will.'

It has been eight weeks since Harry, 19, was killed in a case which has made headlines around the world and provoked an outpouring of support and sympathy for his devoted family. The details are now well known.

On August 27, Harry's motorbike was involved in a head-on collision with a 4x4 driven by Anne Sacoolas, the wife of a US spy, who was allegedly driving on the wrong side of the road after leaving RAF Croughton, a US intelligence hub in Northamptonshire.

The grief and shock which kept Niall (right, alongside brother Harry in matching Noddy pyjamas) house-bound for weeks after the death of his brother has now turned to anger, which he is more easily able to express

Within days, Mrs Sacoolas had fled back to the States with her husband Jonathan and their three children, claiming diplomatic immunity. The Americans have repeatedly refused to issue a waiver, despite the family taking their campaign as far as the White House, where they unsuccessfully appealed to President Donald Trump.

Compounding their grief is their anger at the way the Foreign Office and Northamptonshire Police have dealt with the matter. They are seeking a judicial review to establish how Mrs Sacoolas was allowed to leave the country so easily.

Niall has, until now, felt unable to join in his family's campaign for justice. He is naturally reticent and, he admits, socially awkward, the very opposite of his extrovert twin brother.

But the grief and shock which has kept him house-bound for weeks has now turned to anger, which he is more easily able to express.

Harry (left) and Niall (right) lived with mother Charlotte and stepdad Bruce after their parents separated when they were 13 months. Niall said he has regrets that the brothers had naturally grown apart as teenagers with different interests

Speaking to The Mail on Sunday, he said: 'I'm not happy with how everything has happened. I've wanted to start grieving but now it's become so huge that I have to be involved.

'When we were told Anne Sacoolas had diplomatic immunity it was a punch in the face. "Tough, she's not coming back, end of story. Go away and cry at home." That's what it really did feel like.

'It was really difficult hearing that and knowing we had nowhere left to go, nothing left to do. They left us to just cry and grieve in silence. I'm angry at the higher-ups of the world who just don't seem to care at all about what's happening.

'It was swept under the carpet and that's what really got to me. It made me angry. Harry lost his life. We've lost everything. I want to know who decided this was OK to do this to us.'

The twins spent much of their childhood zooming about the village of Charlton, Northamptonshire, on BMXs. Pictured: Harry (left), alongside Niall

Appealing directly to Mrs Sacoolas, Niall has begged her to 'do the right thing'.

'We want for this to be over, to live our lives the best way we can. We can't do that unless you cooperate and help us. This all could have been avoided if you had just stayed here and done the right thing. Although that time's passed you can still put an end to all of this.'

Softly-spoken Niall, who works for a steel fabrication firm, is only now gradually beginning to understand what he has lost.

Darker than his twin, there is still a striking physical similarity to the cheeky blond who stands by his side in all of the family's treasured photographs.

Initially he seems unsure whether he will be able to be interviewed at all – he has barely discussed his brother's death yet with anyone.

Appealing directly to Mrs Sacoolas, Niall has begged her to 'do the right thing'. Pictured: Niall (left), alongside Harry, in a primary school photo aged eight

But when he starts talking, he is eloquent, opinionated and tearful.

'We always had so much fun,' he says. 'I do miss basic, simple chats with him about nothing and everything. I didn't realise how much it meant to me at the time. It was good. I was lucky.

'I miss his voice the most. It made me feel at home. He was always cracking a smile. He was so content and it helped everyone else be happy, seeing how easily he took life… He just went for it like it was nothing. That's how he lived every day and I aspire to be like that. I miss him.'

Niall describes his regret that the brothers had naturally grown apart as they became teenagers with different interests.

They had lived with their mother Charlotte and stepdad Bruce after their parents separated when they were 13 months, but remained close to their father Tim and stepmother Tracey.

The two families, including the twins' four step-siblings, lived just a couple of miles from each other.

Birthdays and Christmases were celebrated twice so everyone was involved, and there were holidays to Turkey, Miami, Crete, Paris and Mexico, and an annual seaside trip to Weymouth.

Harry (pictured when he was younger) loved anything on wheels and got his first mini-motorbike aged seven

They rode around the village together on BMX bikes and scooters. Even a move to separate bedrooms aged eight lasted only two nights before they decided they'd rather be together.

'Harry would always smile in pictures while I'd be miserable,' Niall jokes. 'He was loud, obnoxious, funny and happy and I was quiet, boring, sad and unhappy.'

Harry, who loved anything on wheels, got his first mini-motorbike aged seven. At 12, it was upgraded to a dirt bike.

'It worried me, seeing him on it. But he had no fear.'

Otherwise, Harry spent hours on his computer, while Niall was more interested in being with his friends. They rarely spent much time together, but Harry's protective instinct remained.

Since his death Harry's parents, Charlotte Charles and Tim Dunn, have sought a judicial review after claiming that the Foreign and Commonwealth Office's decision to give Anne Sacoolas diplomatic immunity was unlawful

'He knew me very well. He never pushed me to do anything or go anywhere but would be on the sidelines when needed. In shops, I'd pick the stuff I wanted and Harry would go to the till for me as I struggled with things like that.

'At family parties or events I'd sit next to him and if someone talked to me I could say, "Harry, what do you think?" and he'd take over.'

After Harry got his first proper motorbike at 16, there were two accidents – one involving a 4x4 which came round a bend in the middle of the road. 'It was never his fault and both times the other driver admitted liability. Harry saw the 4x4 coming and knew to let go of the bike and throw himself into the grass verge.

'The second time a car pulled out in front of him on a roundabout. That's how accomplished he was as a rider – he knew his limits.' Niall moved out of the family home in April to live with friends in the nearby village of Croughton, a mile from the US intelligence base.

The demands of such a high-profile campaign to bring Anne Sacoolas to justice is delaying the family's ability to grieve

Harry would regularly drop by on one of his rides. The pair would chat about their day and Harry would offer to bring round discounted food from the service station, or give Niall money.

'That sort of thing made him happy. We'd also begun to talk properly again and it was getting really good – we were turning into adults. If I could say anything to him now, it would be to say I'm sorry I didn't appreciate him more while he was here. Those short times we spent together really meant a lot to me.'

On August 27, it had been a beautiful summer's day and Harry dropped in to see Niall shortly after 7pm. He stayed just five minutes, not even taking off his helmet.

'I was in my dressing gown on the sofa and we chatted basic nonsense. I'd hoped he might stay, but he wanted to get off. It was, "All right, mate. Not sure why I'm here, so, see you later."'

Two hours later Niall received several calls from his parents to say Harry had been in an accident. 'I wasn't too concerned. It was, "Oh, another one?" But realising it had happened close by, he grabbed his things and left the house.

'I could see the blue lights in the sky, and ran to the police car which was blocking the main road. Beyond it down the road was a 4x4 and around 20 people, so I knew he was down there. I spoke to a policewoman and said, "That's my brother. What's going on?"

'She said, "Well, he's breathing and he's screaming so he's conscious." She basically said to go to hospital and it would be all right.'

Niall drove with his parents to Oxford's John Radcliffe Hospital, arriving around 10.15pm. 'Even then it didn't feel bad. In a few minutes Harry would come out looking like a mummy and we could laugh at him.'

Growing up, birthdays and Christmases were celebrated twice so everyone was involved, and there were holidays to Turkey, Miami, Crete, Paris and Mexico. Pictured: Charlotte Charles with her son Harry Dunn

But at 11.30pm, their hopes evaporated when the doctors gently told them that, despite the efforts of a 16-strong team, Harry had died. 'It didn't sink in. I started comforting my parents instinctively. I can't tell you how it felt. I still don't think I know.

'They took us to see him, and he was lying on a table with a bit of a bloody lip. His helmet had protected his face, but knowing his body was so broken was one of the worst things about it.

'I called him a silly b*****d. I said, "What are you doing, man? I've got to work in the morning and you're doing this to me." I told him he'd really p***ed me off. He was warm at first. That was difficult. You can't tell me someone's dead when I'm looking straight at them and they're warm.'

Wryly, Niall adds: 'I'd always believed he'd die on his bike. I just thought it would be in 60 years.'

A policeman had confided Harry hadn't been at fault, which had been some comfort. It meant that, when the family promised they'd get justice for him, they naively assumed it would be straightforward. Certainly they have behaved with astonishingly measured dignity given the circumstances.

Mrs Sacoolas was informed of Harry's death by Northamptonshire Police the following day and initially had been remorseful and co-operative.

Police wanted to charge her with death by dangerous driving, which could have meant a custodial sentence. But six days after Harry's death, unaware that there were questions already being raised behind the scenes over Mrs Sacoolas's diplomatic status, the family requested the charge was reduced to death by careless driving, which usually involves a suspended sentence and would avoid separating her from her children.

But in the end, neither was possible. Mrs Sacoolas left the UK around a week later – reportedly on a US military plane – claiming diplomatic immunity. The family were not informed until ten days later, and there are now serious questions over whether the Foreign Office's advice to the police – namely, that Mrs Sacoolas was entitled to such immunity – was correct.

The family has now requested a judicial review into the Foreign Office's decision, and has referred Northamptonshire Police to the Independent Office for Police Conduct over its role in allowing Mrs Sacoolas to leave.

The family's spokesman, Radd Seiger, is travelling to the US today to discuss the possibility of bringing a civil case against Mrs Sacoolas and a separate misconduct action against the American authorities.

The demands of such a high-profile campaign are, undoubtedly, delaying the family's ability to grieve. But at some point, Niall knows they will have to confront the fact Harry has gone.

'We're spending all day, every day, fighting for Harry so it's like he's constantly around us. He'd be ecstatic at what my parents have done.

'I'm thankful they have that strength. They tell me they get it from Harry. I don't think I have any fight or energy – I just plod along following what they do. But staying quiet in the shadows is worse for me than doing good. It's about time I said thank you and got involved. I want the truth.'