A PHOTOGRAPH of Ann Heron's smiling happy face has been the inspiration for successive detectives not to give up the search for her killer.

The apparently motiveless murder prompted the biggest manhunt ever mounted by Durham Police, involving more than 65,000 man hours and 4,500 statements.

The case was even re-examined by John Stalker, the former Deputy Chief Constable of Manchester, for a television documentary, and a Sunday newspaper offered a £10,000 reward for information.

Every lead and every tip-off drew a blank.

But detectives have steadfastly refused to give up on the mystery surrounding the slaying of the 44-year-old housewife.

Keith Readman, the detective superintendent who led the investigation for five years, kept a photograph of Mrs Heron on his desk to act as a constant reminder.

The case became his obsession for five years and he described the scene he found at the house as "gruesome in the extreme".

In the wake of the killing, police chased up thousands of leads, interviewed more than 4,000 people, took 300 sets of fingerprints and spent more than 65,000 man hours searching for a breakthrough.

During this time and ever since, police have repeatedly pleaded for two men who they believed could hold the key to the case to come forward.

But the sun-tanned man spotted speeding down the drive of Aeolian House in a blue car about the time of her murder, and a male jogger, seen running along the A67 near the home, have never been traced.

Two months after her death, BBC1 Crimewatch staged a reconstruction, and later that month, Darlington police received a typed tip-off identifying potential suspects.

Another desperate television appeal six weeks later, this time on Tyne Tees Crimestoppers, resulted in only five further phone calls.

However, less than a year after her death, three Darlington men were arrested in connection with the murder, but later released.

"Looking back, all I can say is I did my best, but unfortunately it was not enough," said Mr Readman.

Despite his disappointment, Mr Readman remained adamant the murderer would one day get his retribution, and said he looked forward to a phone call telling him the killer had been caught.

Mrs Heron's picture was handed over to Detective Superintendent John Blake when he took over the investigation in 1995.

The following summer, police visited an unnamed sex offender in connection with the killing, but the interview came to nothing.

Eight months later, convicted rapist Philip Hann was moved from Wakefield Prison to Darlington to answer questions relating to the case, but was soon ruled out.

Four years later, detectives said they would question convicted killer Philip Smith about the murder but, like all the other leads before, this line of inquiry was soon dropped.

On the 11th anniversary of her death, Det Supt Blake announced that a breakthrough could be "just around the corner".

He said the key to the inquiry would be science.

"There have been tremendous advances in DNA profiling in recent years. While I do not want to raise false hopes, I remain confident the scientists will give us the vital lead we need," he said.

When he retired in 2003, the photograph was passed on again to Detective Superintendent David Jones, who brought the full weight of technology to bear on the case.

In the past year, there have been several key developments in the case.

In July, The Northern Echo revealed detectives were carrying out laboratory tests using the latest forensic technology which could bring them tantalisingly close to finding the killer.

The newspaper also reported that police had commissioned a Yorkshire-based company to reconstruct the Herons' home, at Aeolian House, Morton Palms, using video footage and photographs from the murder scene.

In July, Det Supt Jones told The Northern Echo: "I hope to have results before the end of the year."

The case bears a similarity to the BBC1 series Waking the Dead, where detectives dredge up old crimes and have a fresh look with new technology.

Yesterday, in the most dramatic twist yet in the 15-year manhunt, police swooped on her husband's house in Scotland.

Peter Heron, 70, who once put up a £500 reward for information leading to his wife's killer, was arrested at 7.30am on suspicion of her murder. He was driven with a team of police officers to Spennymoor police station where he was still being questioned last night.

Mrs Heron was born in Scotland and married her first husband, policeman Ralph Cockburn, when she was 18. They had three children, Ralph, Ann Marie and Michael. Their relationship broke up after 15 years.

While she was on a trip to Darlington, she met successful businessman Mr Heron, who offered her a chance for a new and exciting life of holidays overseas and a break from responsibilities.

Soon after her murder, it emerged Mr Heron was having an affair with a local barmaid.

Two years after his wife's death, he married Freda Buddie, who died recently.

In the years which followed her killing, Mr Heron spoke to The Northern Echo several times.

In 1995, he told the newspaper: "I believe I know my wife's killer", saying he was a local man, someone he rubbed shoulders with every day.

The couple's former home is now owned by Andrew and Louise Bloomfield, who run a dog kennels on the site.

The murder weapon, thought to be a Stanley knife or scalpel, has never been recovered.