Detectives still hope that new developments in genetic fingerprinting could hold the key to one of the North-East's most notorious murders.

Hopes were raised last year when police revealed that they planned to carry out DNA tests on materials recovered during Ann Heron murder inquiry more than a decade earlier.

Nine months on the tests have not been carried out.

Instead, police are awaiting further breakthroughs in DNA fingerprinting before they press ahead.

And, although Durham Constabulary's investigators continue to be frustrated that the killer may still be on the loose, a spokesman last night said: "We remain optimistic that there could be a breakthrough at any time."

Mrs Heron was found lying in a pool of blood at her home at Morton Palms between Darlington and Middleton St George on 3 August, 1990.

The case has since become one of the most baffling murders in Britain.

The body of Mrs Heron, 44, was found by her husband, Peter, in the early evening at Aeolian House, just off the A67 on the outskirts of town at Morton Palms.

She had spent the afternoon sunbathing in the grounds of the house before her throat was slashed in what was believed to have been a sexually motivated attack.

Police still believe a sun-tanned man, aged 35 to 40, who was spotted driving a blue car along the drive of the house at about 5pm - when Mrs Heron is thought to have been killed - could hold the key to the mystery.

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

The hope is that forensic science "will ultimately provide the breakthrough" needed by Superintendent John Blake, who leads the investigation.

Late 1999 saw more than 1,500 exhibits collected from the scene were reviewed. A small number have been earmarked for re-examination in Home Office labs, using the latest in DNA technology.

Mr Heron sold Aeolian House last year and has decided to start a new life in Scotland.

The house's new owners, Andrew and Louise Bloomfield, of Yarm, near Darlington, have been granted planning permission to turn the home into an animal training centre and pet crematorium.

Anybody who has any information about the murder is urged to contact the Holmes Room at Durham Constabulary headquarters on 0191-386 4929.