A TIGHTLY-BOLTED door, a spy-hole and a secret knock were not enough to keep the frail divorcee Renee McGowan safe in her top-floor flat.

Mrs McGowan, 55, was divorced two years before her murder on July 23 1975 and, as she lived alone, had done all she could to protect herself from intruders.

Yet someone still managed to get into her top-floor home at Evans Towers, off Manchester Road, and strangle the quiet, well-spoken woman.

She was found partly clothed and with her wrists tied behind her back in her 14th floor flat by her fiance.

Nothing was apparently missing from the flat and there was no sign of forced entry.

Some drawers and cupboards were left open but property of any value and money was untouched.

Mrs McGowan was frail and police admitted she would have been helpless to stop her killer because of a heart condition.

What became known as the “Lonely Hearts” murder due to Mrs McGowan’s connection to the Bradford Phoenix Society, a club for divorced and separated people, quickly turned into one of the police’s most challenging cases.

It was the first murder they investigated in a tower block and detectives soon discovered the difficulties this entailed.

On the surface it looked like the location would help, not hinder, detectives with all of the events taking place in one building.

However, for detectives, it was quite the opposite.

Detective Chief Supt Jack Dibb led the murder investigation and explained the difficulties facing his team in September 1975.

“We have found that there is a veritable army of callers at these blocks,” said Mr Dibb.

“Tradesmen, social workers, insurance people, all need to be seen.

“You might think that with a murder in a tower block the job was made easy as you are able to seal the inquiry off into one building.

“In fact it’s just the reverse. It is much more difficult to investigate a murder such as this.”

“It’s been a revelation to officers working on this case to find how little contact there is between the occupants of a tower block,” he said.

“In each block every floor is like a separate street.

“A person entering the lift on the ground floor to go up to the fourteenth would possibly never see anyone on the floors beneath.”

Mrs McGowan was last seen at 7am on July 22 and police initially called for anyone to contact them who had seen her between 7am and 4pm on that day.

As the days stretched into weeks, police had already interviewed thousands of people, but the strangler was still on the run.

Two weeks after Mrs McGowan’s murder, police hunting the strangler saw the members of the club as key to their investigation, and all of them were asked for their fingerprints.

More than 150 police were still involved in the hunt and Det Chief Supt Dibb was eager to crack the case.

Police had already taken down 500 statements, but urged the members of the Phoenix Society to come and have their palm and fingerprints taken.

They were joined by all 84 occupants of the tower block.

Mrs McGowan's family also came forward to offer a £500 reward for information, but the murder remains unsolved.

One year after the death, officers had interviewed 6,500 people, taken 2,100 statements and recorded 1,000 sets of fingerprints.

Police even spoke to 150 people named in her diaries as they left no stone unturned in their hunt for the strangler.

Mr Dibb said at the time he was "prepared to wait" to catch the killer, having just solved the murder of Bertha Keating in 1964.

He could not have imagined the wait would have gone on for 43 years.

A spokesman for the Force said: "West Yorkshire Police has a dedicated team of detectives who carry out reviews of these investigations in line with new evidence coming to light or advances in forensic technology. 

"If you have information which may assist, then please contact the Major Investigation Review Team on 101 or by using the contact options on the West Yorkshire Police website. 

"Information can also be reported anonymously to the independent charity Crimestoppers on 0800 555111."