The plot has elements of dark farce. The would-be killers crashed as they drove to the scene of their planned crime and later got lost and had to ask for directions.

They made no real attempt to hide an arsenal of weapons packed in their battered old Fiat Punto and they helped detectives enormously by setting out their unlikely scheme to kill Joss Stone – real name Jocelyn Stoker – in a bundle of notes culminating in the unsubtle: "Once Jocelyn's dead … find a river to dump her."

Police accept that Kevin Liverpool and Junior Bradshaw's plot was incompetent and illogical but they are sure it was very dangerous. The pair got within a few miles of her house with the equipment to carry out their plot before they were stopped. And Stone was at her home in rural Devon, relaxing having recently launched her own label, Stone'd Records, and finished her fifth album LP1 in Nashville.

"I have no doubt that Liverpool and Bradshaw were targeting Joss Stone," said Superintendent Steve Parker. "They represented a significant threat to her."

It had already been a long day for Liverpool, 35, and Bradshaw, 32, when they were captured. The pair, childhood friends who were sharing a one-bedroomed flat in the Longsight area of Manchester, left home at 2am on 13 June 2011 in their newly purchased car and headed for the Cullompton area of Devon.

They had armed themselves with a samurai sword along with hammers, a metal spike and knife. They were equipped with balaclava helmets, gloves, body bags, gaffer tape and a roll of plastic bags and had maps showing the area where the soul singer lived, including one marked: "Here Joss Stone."

Liverpool and Bradshaw stopped at a services station on the M5 in Gloucestershire at dawn. They filled up with petrol and drove off the forecourt without paying, another example of their lack of care in covering their tracks. As they left the service station Bradshaw crashed into metal railings and a digger.

The front of the vehicle was badly damaged and Gloucestershire police attended. Officers said they would report the pair for motoring offences but did not believe the vehicle could be moved – and did not know they had left the service station without paying – so departed without searching the car.

The plot could have stopped there. But Liverpool and Bradshaw were nothing if not persistent. They got the car back on the road and arrived in Devon two hours later.

Unhappily for them, the lanes around Stone's home are narrow, complicated and confusing – people who have lived there for years become disorientated.

Joss Stone ran an open house and had no lock on her Devon home, the court was told. Photograph: Ian West/PA

Liverpool and Bradshaw got lost. Desperate, they showed a postman a picture of Stone and one of the men asked him where she lived – another example of their haphazard plan. The postman told them he did not know.

Later, three residents saw the car in Cullompton. They thought the occupants were "agitated and behaving abnormally" and phoned the police. A patrol car followed the Fiat then stopped it. The weapons and equipment were found and the pair were arrested for possessing offensive weapons and going equipped. As police searched the car more carefully – and later the men's flat in Manchester – they realised that something bigger than a robbery had been in the offing.

At the flat were a crossbow and a BB gun. Notes found included the message: "Joscelyn [sic] Stoker … RIP for ever" and: "I don't kill just for dollars, only for good cause or reason." One mentioned decapitation and another read: "Once Jocelyn's dead … find a river to dump her." They also described her as a "she-devil" and seemed to take umbrage at her links to the monarchy (Stone has sung for royals and was invited to Prince William's wedding). "Invited to Will's wedding by Queen. Where's the sense in that?" mused one note.

When she gave evidence, Stone told the jury she had been enjoying a "really nice" day until police banged on her door and told her they believed Liverpool and Bradshaw had been planning to rob her. It got worse when they returned later and told her that, actually, the pair had been planning to kill her.

"That was strange – it was a surprise," she said. "The police came and said these people were trying to kidnap me and in the afternoon of the same day they came back and said no, no, they are trying to kill me."

Stone's girl-next-door-turned-superstar-singer image is no sham. She was born in Kent but has lived in Devon since she was eight. Stone did not live in splendid rock-star isolation but was a liked and respected member of the community. At the time of the plot she was dating a childhood friend.

She ran an "open house". Neighbours were encouraged to pop their head around the kitchen door whenever they wanted. "I've lived in Devon for a long time and nobody really shuts their door," she told the jury. "I had an alarm but I did not really turn it on very much. I didn't really have a lock on my door. Everybody knows where I live. It is that kind of place."

Detective Sergeant Martin Sutcliffe, of Devon and Cornwall's major crime investigation team, said Stone was not at all the "pop star who lives on the top of the hill". "She was brought up in east Devon, she lives in the community and is respected." He said it would be sad if the case meant this changed.

Having told Stone about the plot, police launched Operation Hill to try to find out exactly why the two men had apparently targeted the singer.

The pair have known each other since schooldays in the north of England and called each other "cousin" though they were not related. They were unemployed and lived off benefits. Detectives looked closely at the pair to establish if they had links with organised crime but found nothing. They think others may have been involved but do not believe their strings were being pulled by a gangster or crime baron.

Liverpool has previous convictions for assault and for having bladed articles or knives in public places. On one occasion in 2007 a knife in a holdall along with a ski mask and gloves were found when he was stopped and searched. He was given a community sentence in 2010 with a mental health requirement.

Bradshaw has an IQ level often associated with a learning disability and has suffered from schizophrenia. He has been jailed 11 times for persistently breaching a court order after he was convicted of exposing himself on the steps of Leeds town hall in 2006. Later he appeared at the Old Bailey after being found sleeping rough in London and was given a hospital order that led to him being admitted to a mental health unit, Edale House, in Manchester. He was cared for at the unit between June 2009 and April 2010.

On his discharge he went to stay with Liverpool.

Albert Hoogland, a mental health nurse, said during the trial that concerns had been raised about the arrangement. He said Bradshaw would have benefited from having his own supported accommodation. A consultant forensic psychiatrist, Michael Alcock, who has spoken to Bradshaw since his arrest, told the jury he should have been monitored professionally after leaving the unit.

Bradshaw stopped taking his medication a few months after leaving Edale House and his health deteriorated.

By January 2011, police believe, the conspiracy was being hatched. The pair had no computer at the flat. Instead Liverpool used a computer at his local library to research various artists, from Girls Aloud to Beyoncé, Dizzee Rascal and Eminem, before they fixed on Stone. Liverpool appears to be the leading light and set about working out where she lived, printing out maps and trying to find a car and weapons, and put together their kidnap kit.

Texts to contacts in Manchester found by the police showed Liverpool had tried to source a semi-automatic gun, silencer and night vision kit. A text sent on 12 January to a third man read: "Make sure you have a car and a piece for me. Semi Automatic preferred."

On 15 January he texted: "Yo, I'm looking to take out Jocelyn now and set it off. Get them round to me tomorrow. Let's do this thing, let's make this move." On 13 February he texted: "Make sure you have got a car and three guns. Semi autos. Infra red. Silencer."

Bradshaw's fingerprints were not found on any of the weapons nor could any text messages or notes relating to the plot be attributed to him directly.

He told the jury he had not even heard of Joss Stone before he was arrested and was just on a "day out".

By May the red Punto had been bought and the police believe that at the end of the month they carried out a reconnaissance trip to the village where Stone lived. A neighbour saw a red car driving slowly past Stone's home.

Then came the chaotic day when they tried to put their plan into effect. Three different people dialled 999 after seeing the men behaving suspiciously and the plan was halted.

The pair refused to tell police what their motivation was. They did not appear to be fixated on Stone – there was no shrine to her at the Longsight flat and they did not even have copies of her CDs. There may have been a financial element.

They appeared to be interested in her royal connections. Asked about her links to the monarchy, Stone said: "I have sung for them a couple of times – once for the Princess Diana concert. After that there was another charity event for [Prince] Harry." She was invited to Prince William's wedding. "I guess they just invited me because they thought it was nice to invite me," she said.

Police emphasise there seemed to be no logic to the plot. The pair were not under the influence of drink or drugs but were jumbled, disorganised and unprofessional. One note read simply: "Destroy the Queen's system." But more than anything, they were dangerous. Stone told the court she did not have a proper lock on her door or set her alarm. "But I do now," she added.