It was the most innocent of excursions. Thomas Devlin and two friends had gone to the garage to buy sweets, crisps and Lucozade. It was a balmy summer's evening and they were enjoying the school holidays, laughing and joking, as they walked back to Thomas's home to play computer games

They were only 200 yards from the house when the killers struck. The boys were easy prey – younger and with far less street sense than their assailants. At the end of the brutal attack with a knife and a two-foot-long wooden bat, Thomas Devlin (15) lay fatally stabbed and his friend Johnny McKee badly beaten.

Only Fintan Maguire escaped unhurt. The leafy, mainly Catholic Somerton Road in north Belfast is home to solicitors, teachers and other professionals. Even during the worst days of the Troubles, it wasn't somewhere associated with random murder.

The killers had come from the loyalist Mount Vernon estate, about a mile away. Within weeks of the murder in August 2005, police knew their identities, as did Thomas's parents from their own enquiries. Yet it took five years and an uphill battle with the Public Prosecution Service (PPS) before the family got justice.

"The PPS twice refused to prosecute my son's murderers," says Penny Holloway. "It was only when independent counsel in England reviewed the case that we secured a prosecution. Without our persistence, Thomas's killers would still be on the streets."

Nigel Brown (26) and Gary Taylor (23) were convicted in February and sentenced to life imprisonment. "Thomas's case showed that the PPS needs a complete overhaul but, four months later, nothing has happened," says Penny. "The PPS hasn't even indicated that it did anything wrong. The pain of grieving for our son was compounded by the stress of dealing with the PPS yet we're still awaiting an apology.

"What's most worrying is that with the devolution of policing and justice from Westminster to Stormont, the PPS is actually more unaccountable now than when Thomas was murdered. What happened to us could easily happen another family and they would have even less options to challenge the PPS than we had."

The Devlins are a well-educated, articulate couple. Penny works in industrial relations. Her husband, Jim Devlin, was a BBC engineer. They're concerned that a catalogue of similarly wrong decisions may have been made by the PPS in the past but other bereaved families have lacked the knowledge or resources to challenge them.

Gentle and generous

His friends called him 'Dev'. In family photographs, Thomas Devlin seems younger than his years. Those who knew him speak of a gentle, generous, well-mannered lad. He played the tenor horn at school but he loved heavy metal music too. Thomas was the youngest of three children. He had rabbits, guinea pigs, hamsters, mice and a Great Dane called Rosie. He'd built a treehouse at his grandparents' farm.

"He was always late for school because he didn't like getting up in the mornings," says Penny. "But he was a good pupil. He wanted to study law or computer science at university. He was our brilliant, beautiful boy. He loved to bake. His grandmother taught him to make pancakes and he'd just perfected a recipe from the internet for chocolate brownies."

Penny laughs as she remembers coming home from work to find the bathroom and every towel in the house a mess – and Thomas's hair dyed red. "From a young age, he was very funny. He had some great one-liners. He called his father's bald spot 'Dad's sun roof'."

Thomas was brought up Catholic in a religiously mixed marriage. He had friends from both communities. He was a member of the local Boys' Brigade run by the Presbyterian church. He'd go to meetings with his BB uniform under his goth clothes. On Saturdays, he and his mates would head to Belfast City Hall where goths, punks and skaters gathered.

Thomas's killers, Nigel Brown and Gary Taylor, inhabited a very different world. Brown had 72 previous convictions, mainly for violence and disorder. Taylor had 19 convictions for similar offences and the court heard he held "deeply sectarian attitudes". Both men had been engaged in sectarian street fighting in north Belfast in 2003.

Taylor was armed with a knife and Brown with the wooden bat when they left Mount Vernon, with Brown's dog Zola, on the night of Thomas's murder. They were walking along the Somerton Road when they spotted the three teenagers. For some reason, Thomas sensed danger. "Run!" he shouted to his friends.

Fintan Maguire scrambled to safety over a school gate. Thomas tried to make it but didn't. Taylor stabbed him nine times in the chest, abdomen, arm and face. From the other side of the gate, Fintan could hear his friend screaming in agony. As Taylor stabbed Thomas, Brown beat Johnny McKee with the wooden bat. When Taylor had finished with Thomas, he then stabbed Johnny.

But Johnny was better protected. Thomas had been carrying a rucksack but had a sore back and had asked his friend to take it. As Taylor stabbed Johnny in the back, the rucksack probably saved his life. Taylor and Brown walked off, leaving their two victims for dead.

As local man Lawrence Kelly passed them they covered their faces and Taylor threatened: "We'll do you too." Thomas's parents were having a meal at a nearby hotel when they learned of the attack. They ran to the scene.

"Thomas was lying, breathing heavily with a vacant look in his eyes and a paramedic desperately trying to save his life," says Penny. "He was taken to hospital and we stood outside the theatre. But when I saw someone leave and then return with clean sheets, I knew Thomas was dead."

The big breakthrough in the murder investigation came within weeks. Brown told a relative details of the attack. That individual told a relative of theirs who is a serving PSNI officer. But the individual Brown personally told refused to make a statement to police. The Devlins want this person, and four others, prosecuted for withholding information. The PPS hasn't announced its decision.

In July 2008, three years after the murder, the PPS told the Devlins that, after examining police files, there was insufficient evidence for a reasonable prospect of convictions. The PPS had sought advice on the case from one of the North's leading prosecutors, Gordon Kerr.

"The PPS never told us we were entitled to appeal its decision," says Penny. "We only discovered that by trawling through the internet. We requested an external review because the legal fraternity in Northern Ireland is so small we believed we were unlikely to get an opinion going against Gordon Kerr's."

The Devlins now had hired solicitor, Joe Rice, who fought their corner with gusto. They also continued doing their own research. From studying fatal stabbing cases in England and Wales, Penny found that the Crown Prosecution Service there took a much more assertive approach than the PPS. "We were certain that if Thomas had been stabbed in England or Wales, his killers would have been quickly in the dock," she says.

In August 2008, the PPS announced an internal review into the Devlin case by senior assistant director Raymond Kitson. Three months later, Kitson rubber-stamped the original decision: Brown and Taylor wouldn't be prosecuted for Thomas's murder.

The Devlins wrote to the then British attorney general, Baroness Scotland, who had a supervisory role over the PPS. Within 72 hours, there was a u-turn. The PPS now agreed to seek the advice of senior counsel in England David Perry. He concluded that there was "compelling circumstantial evidence" against Brown and Taylor and that the test for prosecution was met. English barrister Toby Hedworth was brought in to prosecute. It took the jury just 80 minutes to reach guilty verdicts.

Failure to prosecute

The Devlins' solicitor, Joe Rice, says: "How did independent counsel in England arrive at a decision on prosecution so substantially different from the PPS? Nothing had changed in the case, there was no new evidence, no new witnesses. Why was a jury able to convict so quickly in a case which the PPS initially didn't think met the prosecution test?

"The PPS hasn't answered any of these questions. It owes an explanation to the Devlin family and to the public. There are serious implications for previous cases before the PPS. How many other killers are possibly free because of its failure to prosecute? How many bereaved families, without the knowledge and determination of the Devlins, have been denied justice?"

Rice says legal experts from Dublin and London must be brought in to regularly evaluate the decisions of the PPS and the opinions of prosecuting counsel in the North. The Devlins are completely satisfied with the PSNI's conduct: "One reason we persisted in fighting for prosecutions was because of the PSNI's total belief in their case files." Sources say detectives themselves were "very frustrated" by the PPS's attitude.

Although they secured justice, the Devlins know they haven't "won". Thomas's bedroom remains just the way he left it with his Iron Maiden posters on the wall and science fiction novels lining the shelves. His parents have kept all his school work.

He had always wanted a balcony for his bedroom. His father was building it when Thomas was murdered. It's finished now and, on sunny mornings, Jim and Penny sit there drinking coffee, thinking of their son. "Thomas would have been 21 in September," says Penny. "We would have thrown him a great party. His friends always call on his anniversary.

"We hear about their girlfriends, their travelling, and how they're getting on at university. It's comforting to chat to them but it hurts so much too. It brings home all the things that Thomas has been denied. Our battle for justice was successful but what we really want is to have Thomas alive at home with us."

In a statement to the Sunday Tribune, the PPS acknowledged "the tenacity" of the Devlins and apologised for "any additional stress" they experienced during the review of the original decision not to prosecute.

It hoped the successful prosecution of Brown and Taylor "in what was a difficult circumstantial case, will bring some comfort to the family".

The PPS said it had previously met the Devlins five times to discuss "prosecutorial decisions" and intended to meet the family again. It denied that the devolution of policing and justice to Stormont meant it was now even less accountable than before.

"The new arrangements mirror the arrangements which have existed in the Irish Republic since 1975 when the office of the Director of Public Prosecutions was established," a spokeswoman said.