He was just 21 and a budding TV executive. She was a beautiful 19-year-old medical student with whom he’d fallen desperately in love.

In February 2011 Gagandip Singh drove off from his family’s house in south-east London to Brighton to visit her. He would never come home.

‘I remember Mum saying he wasn’t exactly dressed for a party because he was wearing a hoodie,’ says his sister, Amandip Singh, recalling how he’d told their mother he was going to a party and would be home late.

The following morning Singh’s body was discovered in the boot of the burnt-out Mercedes he had borrowed from his sister to make the 62-mile journey. He had been brutally beaten before being dumped in the car and set alight.

This revelation would be shattering enough for any family but the true story was darker still. In a complex and unsettling tale that would later emerge in harrowing detail during a three-month court case, he had been lured to his death by the woman he was so besotted with, Mundill Mahil.

Her motive, prosecutors argued, was vengeance: six months earlier, she had told friends of her devastation that Singh had sexually assaulted her. One of those friends, Harinder Shoker had, along with an accomplice, vowed to teach Singh a ‘lesson’.

Then 20, Shoker was later sentenced to life for the murder, while Mahil was given six years in prison for GBH with intent after a jury found that she was not guilty of his murder but had lured him to his death.

Yet, as a compelling Channel 4 drama which recreates the events leading up to Singh’s death reveals, the reality is more nuanced than a mere revenge plot or ‘honeytrap’.

Both Singh and Shoker – who had known each other since their early teens – were besotted with Mahil. She knew Shoker having met him at a Sikh martial arts club and was subsequently befriended on Facebook by Singh.

It created a toxic love triangle, made more combustible still by the backdrop of the devout Sikh community against which it unfolded, in which honour and purity loom large.

All these themes are delicately portrayed in a series which is sensitive to Mahil’s anguish about Singh’s murder while not shying away from the extent to which she ignored his desperate pleas for help when he was being beaten. ‘One of the things that is hardest for our family is knowing that her name was the last word my brother spoke,’ says Amandip.

‘We know he was calling for her [to rescue him] but she did nothing to help him.’

It is 15 years since Gagandip Singh was murdered, snatching away a promising future for the TV executive who, only a year before his death, had set up a TV channel for Sikh viewers. ‘That was going to be his life’s work,’ says Amandip, now 33 and a married mum of three. ‘He felt Sikhs didn’t have a voice and was passionate about giving them a platform.’

Initially however, her brother had not been especially devout. Raised in Bexleyheath, south-east London, Amandip recalls him as a ‘typical’ boy who loved football and his PlayStation.

A visit to India in his teens proved a turning point, however.

‘He became a baptised Sikh and started practising in earnest,’ Amandip recalls. It was his renewed devotion that inspired him to launch the digital channel Sikh TV in October 2010.

By then, the Singh family had already been marked by tragedy: a year earlier their father, who alongside his wife ran a successful recruitment company, was murdered while on a business trip to India at the age of just 42.

‘It was incredibly hard for all of us,’ Amandip says. ‘It changed the landscape of our family.’

Profoundly affected by the loss of his father, it was to a new friend that Singh increasingly turned: a month before the tragedy he had connected on Facebook with Mundill Mahil, a striking Sikh girl from Rochester, Kent, who was destined for medical school and hoped one day to work for Medecins Sans Frontieres.

The pair shared a mutual friend: Shoker, a trainee electrician and fellow Sikh who, it would later emerge in court, Mahil called her ‘gangster’ friend because of his streetwise manner and cannabis habit.

It is a description Amandip struggles to recognise, despite Shoker’s role in her brother’s death. ‘I knew him well when he was younger as he spent a lot of time at my house. The man I met was shy and struggled to meet your eye,’ she says.

Whatever bond Singh had with his old friend, however, was increasingly threatened by his burgeoning friendship with Mahil.

‘At home he was quite bottled up about what had happened to our dad. Mundill was the one he confided in. She was a shoulder to cry on,’ Amandip says, recalling that while she did not meet her, her brother had insisted she speak to her on the phone. ‘My brother didn’t talk about relationships, but I knew that meant she was obviously someone special to him,’ she says.

 Her mother, however, did meet Mahil once. ‘She came to drop off a jacket belonging to Gagandip and then they sat outside talking in the car for about an hour,’ Amandip recalls. ‘It was quite late, which my mum didn’t like. She thought it wasn’t the behaviour of a good Sikh girl.’

Nonetheless, the friendship between the young students deepened, although Mahil would later claim it had become increasingly one-sided. By now ensconced in a student house in Brighton, where she had moved to study medicine after securing three A grades at A-level, Singh would frequently turn up unannounced.

The dynamic between them had also shifted: Singh had told Mahil he loved her and, while she insisted she wanted only friendship, he continued to hope her feelings might change.

He was not the only one to pursue her. As the Old Bailey would later hear, Shoker, too, had become infatuated with Mahil and increasingly jealous of Singh’s closeness to her.

Then, in August 2010, came the night that would alter the course of all their lives.

Mahil was alone at her Brighton house revising for an exam when Singh turned up unexpectedly. Despite her protests that she needed to study, he asked to stay.

Reluctantly, she agreed – a decision that would have fatal consequences. After falling asleep on her bed, exhausted, Mahil woke to find her friend on top of her, pinning down her wrists as he attempted to rape her.

In a newspaper interview after her release from jail, she recalled seeing his kirpan – the ceremonial dagger baptised Sikhs wear – and kachera, the traditional undergarments intended to symbolise sexual purity. ‘It was a completely different person, like Jekyll and Hyde,’ she said. ‘I was trying to struggle all the time. I felt so weak... when he took the kachera off, I had this burst of energy and managed to fight him off.’

Singh was immediately remorseful but the damage had been done, although, constrained by her faith, Mahil felt unable to go to the police. ‘I thought it was my fault. I let him stay – no good Sikh girl does that. What if my family found out? What if the community found out? Who would believe me anyway? They would say I was leading him on,’ she recalled.

Amandip remains measured when discussing what she believes happened that night. ‘Only two people know what really happened in that room and one of them is dead,’ she says.

That Singh was desperately sorry for what unfolded is not in dispute: he texted a friend confiding what he had done and in the weeks that followed bombarded Mahil with texts and calls, pleading for forgiveness.

In the new drama, Mahil is shown to be increasingly distressed by the volume of contact – and while she felt unable to tell her family about what had happened, she had confided in a furious Shoker who, as the Old Bailey later heard from the prosecuting counsel, ‘wished to exact revenge’.

Mahil has always insisted she had discouraged any intervention from Shoker – but by February 2011 that position had changed. By then, Shoker had told her that, through another acquaintance – an older man named Sonny – he had heard that Singh had behaved inappropriately towards other young women and needed to be ‘taught a lesson’.

‘[Shoker] made the allegation but there was no evidence of this at all,’ says Amandip.

Either way, with Singh no longer answering Shoker’s calls, only Mahil could broker the meeting Shoker demanded. The consequences of this are recreated with unflinching clarity in the drama.

After contacting Singh and instructing him to arrive at her house no earlier than 11pm, Mahil drove to the train station to collect Shoker and – she believed – Sonny. She has always maintained she believed their intention was merely to confront Singh.

Yet when she arrived at the station, she found Shoker accompanied by a non-Sikh called Darren Peters, an old school friend with convictions for car theft.

By then Singh was already on his way in his sister’s Mercedes – and he arrived at what can only be described as an ambush.

Once inside Mahil’s room, he was savagely beaten by the two men, as Mahil fled to her housemates for comfort.

Police reports suggested the attack lasted around 20 minutes, although she later insisted it had been closer to five. In the TV drama, one flatmate is shown trying to intervene, only for Mahil to stop her – a reaction she later explained as down to trauma and terror.

‘Even if she didn’t know exactly what was going to happen, she did nothing to stop it once it started,’ says Amandip, fighting tears.

Mahil later claimed that after hearing the men leave, she phoned Shoker, who reassured her that Singh was alive and unharmed.

He was still breathing – but only just. He had been wrapped in a duvet from Mahil’s bed and bundled into the boot of the Mercedes, his hands tied with cable.

The two men then drove the car to Blackheath, south-east London, where they abandoned it after setting it alight. Police discovered the blazing vehicle in the early hours and, after tracing the registration, arrived at the Singh family home not long after dawn.

Today, Amandip vividly recalls the horror of being woken by police at the front door. ‘They asked if we knew where Gagandip was and I just kept saying: “He’s gone to a party.” I wasn’t thinking straight,’ she says. ‘At this point they didn’t say what had happened to him. It sounded like he had done something and they were trying to track him down.’

After frantically phoning friends, Amandip learned her brother had gone to Mahil’s house and called her to ask if she knew where he was. ‘At first she denied he had even been there,’ she says. ‘Then when I told her I knew he had been there, she said: “Oh, he did come but he didn’t come inside.” So that was two lies within 60 seconds.’

It was not until later that afternoon however that the family learned the devastating truth: Singh’s body was inside the burnt-out shell of Amandip’s car.

‘I was numb. I went straight into shock,’ says Amandip now. That shock was exacerbated by the news that within two days police had arrested Shoker, Peters and Mahil and charged them with her brother’s murder. ‘I couldn’t make sense of it,’ she says.

It would take nearly a year, and the trial of the three defendants at the Old Bailey before Amandip and her mother learned the full horror of Singh’s fate – not least that he had still been alive when he was put in the boot. ‘He actually died from asphyxiation from smoke inhalation,’ says Amandip.

Giving evidence Shoker, who denied murder, claimed events had spiralled out of control and that the original plan had simply been to take Singh back to London to meet Sonny. Mahil, meanwhile, insisted she never believed there would be serious violence. ‘Not even a slap?’ she was asked in court. ‘Maybe a slap,’ she said.

Amandip rejects her version of events entirely, pointing out that one of the first things Mahil did the following morning was buy a replacement duvet. ‘She then carried on with her normal life,’ she says. ‘I will always believe she knew something terrible was going to happen that night.’

That belief was shared by the jury who, while choosing to acquit Mahil of murder, found her guilty of GBH with intent. Shoker and Peters, who were respectively found guilty of murder and manslaughter, received 22 years and 12 years behind bars.

Today, the Singh family remains angry that Mahil was not convicted of murder. ‘While she was not physically there when he died, as far as I’m concerned she is responsible,’ says Amandip.

Mahil was released on licence in 2014, after serving half of her six-year sentence. Unable to become a doctor because of her criminal record, she now works for a charity that helps women with criminal convictions find jobs.

Mahil has since spoken publicly about her youth and naivety at the time of Singh’s murder, but Amandip has little patience for such explanations. ‘My brother was also young and his life was taken from him,’ she says. ‘She went to prison but now she is out living her life. She has never tried to reach out or apologise in any way.’

The impact of her brother’s loss remains devastating. ‘Mum struggles every day,’ Amandip says, her eyes filling with tears. ‘She takes comfort from my children but they are also a reminder of what she lost – the children Gagandip will never have. Whatever happened, I want people to remember him for the good person he was.’

Vengeance: Murder On The Heath is on Channel 4 on Sunday at 9pm. Both episodes will be streaming on C4 straight after episode one.