Early on the morning of 17 April 2011, Robert Trigg left the flat in Worthing that he shared with his girlfriend, Susan Nicholson, and walked to the newsagent for a pack of cigarettes. On his way home, he lit one. Then he called his brother Michael and told him that Susan was dead.

Michael urged him to call the police. Instead, he called a neighbour, Hannah Cooper, and told her the same thing. Cooper also said he should call the police. In the end, she stood with him and made the 999 call on his behalf. Much later, the police released the audio recording of the call. “He phoned me just now, saying: ‘I think Sue’s dead,’” Cooper tells the dispatcher. “So I’ve come down and he says she is.” She then asks Trigg what happened. You can hear him in the background of the call. “I think … could be suffocation,” he says. On the recording, his voice is flat and expressionless. He sounds like a man in shock, struggling for words.

When the police arrived, Trigg told them a story. At the end of a night of heavy drinking, he and Susan had fallen asleep on the sofa, he said. He had woken up to find himself on top of her. Her face was purple. He thought she was dead, he told the police. But he panicked and so went to the shops.

There were no signs of a struggle, and panic makes people do strange things. Trigg’s story seemed to be consistent with Susan’s condition. Two days later, the pathologist, Simon Poole, conducted a postmortem and his findings confirmed it: the cause of death was likely to have been a mix of accidental smothering and the effects of intoxication. The police concluded that the death was not suspicious.

Six years later, sitting in the front room of their seaside home in Worthing, Susan’s parents, Peter and Elizabeth Skelton, remember the moment they learned of their daughter’s loss. It had fallen to their grandchildren – Joseph, now 30, and Marc, now 28, from Susan’s previous marriage – to give them the news. “The police had told them their mum had died in her sleep,” Peter says. “We just had to believe it. We were devastated.”

“You think: ‘That can’t be true,’” Elizabeth says. “And you think back to when you saw her and she looked fine. Why would she be dead?”

Surrounded by pictures of their beloved grandchildren, the Skeltons speak quietly and thoughtfully, taking their time with each answer. They are 81 and 82 now and seem stoical in the face of loss. But, occasionally, their eyes glaze over. If they seem a little absent in these moments, it is easy to see why. The question of why their daughter died took far longer to answer than they could have expected – and came at a far greater cost.

Susan Nicholson … her marriage broke down and she met Trigg in a rehab clinic.

Susan Nicholson’s life had not been easy in the years before she died. After a decade working at banks and a utility company in London and Sussex, she had moved to Worthing to be with her husband, with whom she would have Joseph and Marc. But the marriage broke down and she started to drink heavily. In 2010, she went to a rehabilitation clinic, where she met Trigg. By the end of the year, things seemed to be looking up. She bought a flat and her new boyfriend moved in. He even met her parents – only two or three times – but he seemed friendly, the Skeltons say.

But after Susan’s death, something about Trigg bothered Peter and Elizabeth. He was 5ft 9in, with a large, imposing frame, and weighed 15st. And they had seen the sofa. It was small – far too small for the couple to sleep on together, they thought. It just didn’t make sense.

Elizabeth phoned DI Sarah Barrett, one of the officers who had attended the scene. “I said, ‘Have you seen the width of the sofa?’” she remembers. “There’s no way two people could sleep on that sofa.”

They pushed the detective, Elizabeth says, for an answer. But Barrett’s mind was made up. And the postmortem’s conclusions were clear enough. On 14 October, they buried their daughter. Even with an inquest still to come, the case of Susan’s death had been closed.

And then, a couple of months later, just before the inquest, the Skeltons were told something that would change their lives. Police informed them that Trigg had been cautioned for assaulting Nicholson. But, more disturbingly, they were also told that years earlier Trigg had been in a relationship with a woman who had died suddenly in the bed they shared. And the police knew about it.

On the day she died in early 2006, Caroline Devlin, who had moved to Worthing from Scotland with her four children, had something to be excited about: she was about to start a job at the probation service. Her children – Jordan, 14, Codie, 10 and Brandyn, nine – wanted to do something for her. It was Mother’s Day and they were planning to make her breakfast.

The family shared a house with Trigg, who Devlin had been with for a couple of years. But when Jordan came out of his bedroom, he found Trigg sitting at the top of the stairs leading to their loft bedroom, fully dressed, clasping a cup of tea. Trigg casually told the boy: “There’s something wrong with your mum.”

Trigg stayed where he was, drinking his tea, as Jordan went past him into the bedroom. He found his mother lying face down with her head at the bottom of the bed, naked. He tried to wake her up. But she was cold and her face was blue.

The police arrived soon after. They knew that Trigg had been cautioned for assaulting another former girlfriend, Susan Holland, whom he had dated in 2003. But they did not see Devlin’s death as suspicious. A postmortem at Worthing hospital by Barbara Borek, a forensic pathologist, concluded that the 35-year-old had died of an aneurysm. The death was put down to natural causes, and no inquest was held.

Trigg stayed on at the house for a short time and then disappeared from the lives of the children Devlin had left behind. It would be another four years until he met Susan Nicholson. When he did, he started abusing her within a matter of months. This was recorded, too. Hannah Cooper, the neighbour who called the police when Trigg told her that Nicholson had died, had made several calls to Sussex police reporting disturbances in Nicholson’s flat.

At least one of those calls was serious enough to set alarm bells ringing. On 26 March 2011, Cooper told the police she could hear trouble from Nicholson’s flat. When officers arrived at about 11.30pm, they found Nicholson with visible injuries to her face – two black eyes and a swollen nose and mouth. She had bruising to the top of her chest and a cut on her right arm. The next day, Trigg was cautioned for battery. But no one made a connection to what had happened to Caroline Devlin.

Even at Nicholson’s inquest, no one questioned the manner of Devlin’s death. She was mentioned, and a single incident of violence against Nicholson was briefly discussed. But there was no reference to the attack that had been bad enough to put Holland into hospital for three weeks.

At the inquest, Elizabeth Skelton told the coroner, Michael Kendall, she could not accept the conclusion. The couple questioned the size of the sofa and challenged Trigg over his violent behaviour. “I just have to say we’re not at all happy,” Elizabeth told the coroner. “I don’t think the truth has been told at all.”

But Kendall was unmoved. He told the Skeltons: “I’m sorry the proceedings don’t dot every ‘I’ and cross every ‘T’ and that’s very unsatisfactory for both of you, but, as far as my jurisdiction is concerned, I feel that I’ve heard the evidence that I need to hear.” He recorded a verdict of accidental death.

The Skeltons say they felt as if their own daughter had been put on trial. References to Susan’s drinking and patchy employment history were made. “But she wasn’t there to defend herself,” Elizabeth says. “We knew it was a sham. It really was.”

At the time of the inquest, the Skeltons had no idea their daughter was being regularly abused. They glaze over – particularly Peter – when they are asked about this period of their daughter’s life.

They had first met Trigg at a charity shop that Nicholson was looking after for its owner. It was 11am. Peter says, “I wouldn’t say he could hardly walk, but he seemed very drunk. He had certainly been drinking. We weren’t pleased with that.” A week or so later, back in the shop, they were warned by its owner: “That man will bleed Susan dry.”

Still, Trigg was polite and affable. Looking back, however, they see signs of what was really going on. Elizabeth recalls an odd conversation that took place about two weeks before Nicholson died. Without prompting, she announced that she had told Trigg that if “anything happens” to her the “flat goes to Joe and Marc”, her sons. This would have been shortly after Trigg was cautioned for battery. Elizabeth reflects on this chronology. Then, hopelessly, she says: “The following Sunday, she was dead.”

In early 2012, the Skeltons, then in their mid-70s, were shattered. But they couldn’t accept the findings. So, on 12 January 2012, they wrote to the chief constable of Sussex police, Martin Richards. He replied seven days later, saying he had referred the inquiry to another officer.

Then in March, an article under Trigg’s name appeared in Take a Break magazine. Under the headline “Killed By a Cuddle”, it told a story of terrible grief. “She had beautiful, bright-blue eyes, and from the moment I met her, all I wanted to do was make her smile,” it read. “She didn’t move, and I started to panic. I held her hand and said: ‘Susan, please come back. Don’t leave me, not now.’”

The article concludes: “I’m trying to come to terms with what happened. As long as I still have my precious memories, I believe Susan will always be with me.”

The Skeltons were astonished. And they were as convinced as ever that there had been foul play. After an inconclusive meeting with DI Barrett, they resolved that if the police would not pursue justice for their daughter, they would have to do so themselves. The details of their efforts are painstakingly recorded in a light-brown folder marked “Susan”. In May 2012, they wrote to the Independent Police Complaints Commission (IPCC). The IPCC recommended that Sussex police investigate the complaint and review the investigation. It was a glimmer of hope, but it wasn’t to last. They received a report dated 3 October 2012 by DCI David Wardley-Wilkins, which concluded the investigation had been satisfactory. There was no mention of Trigg’s history of violence, nor of Devlin’s death.

The Skeltons tried again. On 8 October 2012, they wrote to the IPCC. In May 2013, the IPCC upheld the complaint and, once again, recommended that Sussex police reinvestigate. Again, a Sussex police report, written by DI Emma Brice, concluded that the force had conducted a satisfactory investigation.

But, crucially, this report revealed new details. Brice revealed that PC Adams, one of the two officers to first attend the scene, had been “particularly frustrated” and had asked several colleagues why Trigg had not been arrested. Two days after Nicholson’s death, DI Barrett had spoken with a senior officer about the possibility of arresting Trigg – but decided that it would not be “advantageous” to do so.

The report also acknowledged for the first time that “the previous records of domestic violence” were considered. It revealed Trigg had admitted on the day of Nicholson’s death his arrest for domestic violence four weeks previously. And there was one more extraordinary revelation: on the day of Nicholson’s death, Barrett was aware that Trigg’s previous partner had died – and that, just as with Nicholson, “he did not notice she was dead until the morning”.

The Skeltons were horrified. “They knew when they arrived that he had hit her,” Peter says. “They knew Caroline Devlin died in the same way. They didn’t do anything. We couldn’t believe it. We were devastated.” Once again, the Skeltons wrote to the IPCC to challenge the police’s conclusions. In a handwritten note, dated 28 October 2013, Elizabeth wrote: “Our concerns have increased because we do not believe the police carried out a thorough investigation.”

Even then, there were crucial facts that the Skeltons did not know. No one had told them about Trigg’s previous violence against Susan Holland. No one had told them of his caution for common assault against her, or that the beating that occasioned it was so severe that Holland had to spend weeks in hospital.

In December 2013, the IPCC recommended again that the police reinvestigate. Almost another year later, in October 2014, the Skeltons received another police report declaring the investigation satisfactory.

But once again, the report revealed a little more. The report, by DS Ruth Eady, revealed that one of the officers in attendance – PC Darren Milledge – had been reprimanded for producing a poor scene log when guarding the initial scene. In the log, he names only one paramedic as being present, and fails to record that Trigg and his brother were there.

To the Skeltons’ astonishment, the report also revealed that Milledge had been part of the initial response to Devlin’s death five years previously. The coroner’s officer on Nicholson’s case, responsible for liaising with the police, had also been assigned to the Devlin case.

The Skeltons once again appealed to the IPCC. But this time the police watchdog shared the conclusion of the final Sussex police report that the investigation had been done thoroughly and the family’s complaints answered.

For many, it would have been the end of the fight. But, again, this quiet, unassuming couple vowed to battle on. So in January 2015, after their fourth Christmas without their daughter, they approached a solicitor, Hannah Bennett, who took on the case and appointed a barrister, Matthew Farmer.

“I found it frankly incredible that the police had not properly investigated what on the face of it was a highly suspicious death,” Farmer says. “It just beggared belief the police didn’t treat it as a murder. I had never had a case like it.”

A police photograph of Robert Trigg.

Finally, the Skeltons felt that someone was listening to them. Farmer decided to gather more evidence. He contacted Nathaniel Cary, a Home Office forensic pathologist who had worked on a number of high-profile cases, including the Hillsborough disaster and the poisoning of former Russian spy Alexander Litvinenko. He asked Cary to look again at Nicholson’s and Devlin’s deaths.

Cary’s report was damning. Of Nicholson’s death, he says: “In my opinion … accidental airway obstruction must be the least likely possibility in this case. In terms of the proposed positions on the sofa, this is not a matter of expert evidence but as a matter of common sense there must have been very limited space for two individuals to lie adjacent to one another on the sofa in the manner proposed.”

The Skeltons’ initial suspicions about the sofa were being reinforced by one of the country’s top pathologists. “In my opinion, it is very unlikely that someone asleep on the sofa with another could cause death in the manner proposed by simply rolling on to them,” he went on. “In my career, I have never previously encountered a case where accidental death was thought to have happened in the manner apparently accepted in this case.”

He found that Nicholson’s injuries suggested “traumatic asphyxia”. At last, an expert view of the evidence had emphatically echoed the mountain of circumstantial detail. The Skeltons were not deluded. Their daughter had been suffocated, possibly strangled.

In all the time the police had been rebuffing the Skeltons, Trigg had remained a free man. And in October 2014, he started dating Deirdre Loveridge.

Like his past relationships, this one was rocky from the start. Every couple of months, Trigg would become jealous and angry. In September 2015, Loveridge called it off. In early November, Trigg started hounding her. He sent abusive phone calls and messages, telling her he had visions of her being struck on the head and dying.

Loveridge reported him to the police. On 5 January 2016, Trigg pleaded guilty to an offence of harassment. But it was not until the police received the report from Cary that they decided to reopen the Nicholson case.

In June 2016, DCI Tanya Jones appeared at the Skeltons’ home to inform them a new team had been assembled, and that Nicholson and Devlin’s deaths were to be reinvestigated. “We don’t know why it took them so long,” Peter says now. “But we were very relieved when they told us.”

The fresh investigation commissioned Cary to look at Devlin’s death. He concluded she had not died of an aneurysm, but of a blow to the back of the head. Meanwhile, Trigg started yet another abusive relationship, this time with Caroline Yarwood. It was on 7 November 2016, while he was with Yarwood, that he was finally arrested on suspicion of the murder of Nicholson and Devlin. But, despite his record, he was released on police bail. “We couldn’t understand why,” Peter says. “But we knew he had lied almost from day one.”

While out on police bail, in December 2016, Trigg struck again. On the 20th of that month, he sent Yarwood more than 100 abusive messages, some of which included racial slurs. “You are an Irish cunt, with your Irish black hair and ugly body,” he wrote in one message. On the 21 December, he turned up at the shop where she worked and assaulted her, pushing her against a wall and to the ground in the back of the property.

Trigg was arrested for the assault on Yarwood and, on 23 December, was charged with the offence, along with the murder of Nicholson and manslaughter of Devlin. On 30 March this year, he pleaded guilty to offences of assault by beating, harassment and racially aggravated harassment upon Yarwood. But he denied killing Nicholson and Devlin. His denial failed to convince the jury at Lewes crown court. After a 10-day trial, the jurors convicted the 52-year-old in just six hours.

After six years, the Skeltons had their bittersweet victory: justice for their daughter and for Caroline Devlin. The couple had spent more than £10,000 battling the authorities. Elizabeth Skelton survived a heart attack along the way. The journey, she says, with extraordinary understatement, has been “tiring”.

“You wake up in the middle of the night and you think of something,” she says. “It’s constantly on your mind. It’s always there in your head. I didn’t mind. We had to get it right. We knew we hadn’t been told the truth.”

During the trial, further evidence of Trigg’s history of violence emerged. His police record dates back to the 90s when he was in a relationship with Rebecca Allcorn, with whom he had a daughter, who is now 21. Allcorn ended the relationship in 2002 after what she told a local paper was a “decade” of violent outbursts and drinking.

“The police knew he had a history of violence,” Peter says. “They knew another girl had died in his bed. They knew the width of the sofa. Two people couldn’t sleep on that sofa. It all came out in the trial and the jury took six hours to find him guilty. Why did it take them so long to charge him?”

Peter Skelton is determined to see someone within Sussex police held to account. “I think the officers should be charged with something,” he says. “What they did is more than incompetence. They fobbed us off for six years.”

Despite their anger, the Skeltons do appear to have found some peace. “If at any time in the last six years we had dropped out, he would still be walking the streets of Worthing,” Peter says.

Two days after Trigg was convicted, he was sentenced to life in prison with a minimum term of 25 years. He will be 77 when he becomes eligible for parole. In the meantime, Sussex police has launched three reviews into its handling of both cases. They issued a brief apology after Trigg’s conviction for “not presenting all the facts before the CPS previously”.

Farmer says the couple appear to have had a weight lifted from their shoulders. “You had two quite elderly people totally grief-stricken by their daughter’s death, they wanted justice for Susan and all they got was a brick wall,” he says. “It’s shocking it’s been up to them to fight, fight, fight.”

Devlin’s children, who for so long had believed their mother had died of natural causes, had closure of their own. When Brandyn stood outside court after the conviction and spoke on behalf of his siblings, he acknowledged the debt they owed the Skeltons. “Finally we have got justice after 11 years,” he said. “Finally, as a family, we can put some closure to this. We have always said – it is down to the Skelton family.”