DETECTIVES will today unveil a dramatic new twist in the investigation into one of the North-East's oldest unsolved murders.

Ann Heron was knifed to death while sunbathing at her home near Middleton St George, Darlington, on August 3, 1990, the hottest day of the year.

And today, on the 14th anniversary of her death, detectives will reveal how the murder investigation has thrown up a fresh line of inquiry which they hope may finally lead them to the murderer.

Although officers were staying tight lipped last night, The Northern Echo understands the development is seen as potentially highly significant.

For years, detectives have maintained that they will catch the person who killed 44-year-old Mrs Heron.

The body was discovered by her husband, Peter, lying in a pool of blood at Aeolian House, at Morton Palms. Her throat had been cut.

Last night, Detective Superintendent David Jones, who is in charge of the inquiry, said: "Even after 14 years I believe we can still find the person responsible for this terrible crime."

Police have always felt the murderer must have been sheltered by someone.

Last night, however, Det Supt Jones went further when he said: "I am certain that someone other than the killer knows exactly what happened that day.

"To live with such a crime on your conscience would be an unbelievable burden unless they had discussed it with someone else. Clearly we would like to talk to that someone."

Mrs Heron was attacked - possibly with a scalpel or a Stanley knife - in her garden.

Although part of her clothing had been removed, she had not been sexually assaulted.

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.

Mr Stalker concluded the killer was probably local and was being sheltered by someone.

Detectives believe a suntanned man, aged between 35 and 40 years old, is vital to the inquiry.

He was spotted by two witnesses driving a blue car and speeding down the drive of Aeolian House at about 5pm on the day of the murder, about the time Mrs Heron was killed.

One witness, an engineer, gave detectives a very good description but both accounts were missing two crucial bits of information: they could not say what type of car the man drove and could not remember the licence plate number.

Despite countless appeals, the driver of the blue car has never come forward.

A male jogger, seen running along the A67 road near to the house, has also to be identified.

The case took an extraordinary turn in 1994 when The Northern Echo received a series of letters from someone purporting to be the killer. Police treated the taunts as genuine.

The letters gave experts a chilling insight. They said: "I enjoyed it so much. I love hurting people physically and mentally."

The investigation gained fresh impetus last year when detectives ordered new forensic tests. Det Supt Jones added: "We are continuing to look to develop forensic science techniques to provide accurate results."