Little did Beaulieu river warden Anthony Norris know that his quick snap of a car on a bridge would be the “eureka moment” for police hunting the killer of mum-of-five Pennie Davis.

They had already found a car key beside the 47-year-old's body but its true relevance was a mystery until they were handed this vital photo.

Had it not been for the quick-thinking of Mr Norris, a retired teacher, detectives do not believe they would have solved the case so swiftly.

Speaking exclusively to the Daily Echo, Mr Norris, said: “I am a suspicious person and you rarely see a car on that bridge, so because of my bad memory, I took a photograph of it.

“I never knew it would be important but I am pleased my intervention has helped police in some way.

“I would encourage people to be vigilant and if they see something out of the ordinary to take a photograph because you can't argue with a photograph.”

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Detective Superintendent Paul Barton (above), who headed up the investigation, praised Mr Norris for his actions.

He said: “Initially we weren't sure of the relevance of the key as a few people had been in that field, so it wasn't until we heard about the car on the bridge from Mr Norris that alarm bells started ringing.

“And he didn't just take a description or a note of the registration plate, he had the sense to take a picture of it.

“To have a photo was perfect.”

Now seven months on from that moment cold-blooded killer Robertson and the man who hired him to carry out the execution, Ben Carr, are behind bars for life.

Carr's hatred of his dad's former partner, Pennie, dates back to December 2006 when she made allegations that he had indecently assaulted girls.

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Ben Carr.

Those allegations were never proved but it ruined relationship with his dad, who he didn't see for three years.

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So when Pennie (above), who previously worked for the Daily Echo, threatened to resurrect those allegations eight years later, just weeks before he started a new job with troubled teenagers and less than a month before his dad was due to get remarried, Carr's passionate hatred resurfaced.

He feared the claims would destroy his rebuilt relationship with his dad, ruin his father's upcoming wedding and threaten his own new job, so he needed her silenced “once and for all”.

Knowing he couldn't plunge the knife himself because his grudge against the Sainsbury's supermarket worker was well known, he turned to his drug dealing friend Robertson (pictured below), who had never heard of Pennie until a fortnight before he murdered her.

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The pair had only met several months earlier through their dealings in class A drugs in the Hythe and Holbury area but he knew Robertson had agreed to do anything for cash and knew exactly which buttons to press to convince him of murder.

Over a period of two weeks the pair meticulously planned the murder, with regular meetings and phone calls, as well as carrying out surveillance trips, with Robertson using Maclean's silver Zafira to follow Pennie.

Jurors saw CCTV footage from the Montagu Arms, capturing the wheels of both cars on camera, on both the day of the murder and the day before.

But that cruel plot quickly began to unravel when Robertson dropped the key to co-defendant Samantha Maclean's car he used to get to Leygreen Farm, during the attack and left it beside Pennie's body.

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It was Pennie's new husband Pete, 50, who made the horrifying discovery, who had gone to meet her after work.

But the loss of the key forced Robertson to call a friend to pick him up, leaving Maclean's car on the bridge for several hours, which triggered the suspicions of Mr Norris, who took a picture of it.

When police had the registration number it led them directly to Robertson, as he had been seen using it, and police suddenly had their man.

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Phone records soon connected Robertson (left) to Carr (right), and Carr was arrested.

But Robertson remained on the run for several days, first going to Salisbury and then hiding out in Hythe, as armed units raided the homes of all his friends as the manhunt to find him escalated.

During this time he confessed to not one but two people - conversations which proved key evidence for the prosecution during the trial.

With the net closing in he handed himself in, but insisted he was not the murderer.

It was a lie he stuck to throughout the trial but the case against him and Carr, who told jurors he only wanted her frightened, was too strong for the jury to have any doubt of their guilt.

Reflecting on the investigaton that brought justice for Pennie and her family, Det Supt Barton said it had proved to be one of the more challenging cases for Hampshire police.

Its rural and remote location, chosen purposely by Robertson and Carr, meant that there was little CCTV to rely on, few homes to conduct house-to-house enquiries and limited witnesses in the nearby area.

There also proved to be no forensic evidence to conclusively tie those responsible to the crime, and initially it wasn’t clear from Pennie’s wounds that it was even murder.

Det Supt Barton said: “It has been one of the more challenging cases I have worked on and my initial thoughts were that this was a real whodunit and were we going to be able to find those responsible?”

But thanks to a single photograph police were soon on to Robertson and Carr, leaving detectives to inform Pennie’s family that it was someone they knew.

Det Supt Barton said: “It was a difficult conversation for them because I think they almost wanted it to be someone who they had not known, a mad man rather than someone who had so much hatred for her that they had wanted this.”

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Forensic officers at the field where Pennie's body was found.

A lot was made by the defence that there was little forensic evidence – no DNA, no blood seen on Robertson.

Det Supt Barton added: “Maybe Robertson was extremely clever and protected himself, or he was just lucky, but I think it was the latter.”

Another challenge facing detectives was the lack of a murder weapon.

Pathology reports concluded that the knife was about 12cm long from the tip and about 3.5cm wide – similar to the one Robertson picked out in the kitchen of his friend Natasha Brook, when he confessed.

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Despite fingertip searches by officers, extensive use of police dogs to sniff out any blood and specialist divers trawling the nearby rivers for hours, no knife was found at or around the scene.

But frustratingly Det Supt Barton believes that his team have actually got it after it was recovered from the area where Robertson was living at the time, they were just unable to link any forensics to it.

He said: “During one of our searches at a property in the Mill Pond someone handed us a knife which they found in the street in a bag which matches the description that Natasha Brook gave to police.”