Jocelyn Eve Stoker, better known by her stage name Joss Stone, is a British soul singer, songwriter and actress. Stone rose to fame in late 2003 with her multi-platinum debut album, The Soul Sessions, which made the 2004 Mercury Prize shortlist

One man has been jailed for life and another awaits sentence after being convicted of conspiring to rob and kill the singer Joss Stone in a plot that, at times, bordered on a sinister farce.

Kevin Liverpool, 35, and Junior Bradshaw, 32, planned to decapitate the pop star and dump her body, but were stopped by police within a few miles of her rural Devon home after a series of blunders aroused suspicions. A jury took four hours on Wednesday to convict the two of conspiracy to murder and conspiracy to rob after a three-week trial at Exeter crown court.

Sentencing Liverpool to a minimum 10 years and eight months before being eligible for parole, Judge Francis Gilbert QC told him: "You intended to rob her and kill her and dump her body in the river, according to your words, and then leave the country with your accomplice Junior Bradshaw." Sentencing on Bradshaw was adjourned to a later date.

The trial heard 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 helped detectives enormously by detailing their scheme to kill the singer – real name Jocelyn Stoker – in a bundle of notes culminating in one saying: "Once Jocelyn's dead … find a river to dump her."

On the day of their capture, Stone was at home relaxing. She had recently launched her own label, Stone'd Records, and had finished her fifth album LP1 in Nashville.

The unemployed assailants – childhood friends who shared a one-bedroom 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 were armed with a samurai sword, hammers, a metal spike and knife and equipped with balaclavas, gloves, body bags, gaffer tape and a roll of plastic bags. They also had maps showing the area where the singer lived, including one marked: "Here Joss Stone."

Stopping at a service station on the M5 in Gloucestershire at dawn, they filled up with petrol and drove off without paying. As they left the service station Bradshaw crashed into metal railings and a mechanical 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 previously 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. However the lanes around Stone's home are narrow, complicated and confusing, and they got lost. In desperation they showed a postman a picture of Stone asking him where she lived, but he told them he did not know.

Later, three residents seeing them in their car appearing "agitated and behaving abnormally" all called police. A patrol car stopped the Fiat. 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, a BB gun, and notes, one of which read: "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 took umbrage at her links to the monarchy (Stone has sung for the 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.

The two refused to reveal their motivation. They did not appear to be fixated on Stone, and they did not even have copies of her CDs. Detectives have not ruled out others may have been involved.

Liverpool has previous convictions for assault and having bladed articles or knives in public places. In 2010 he was given a community sentence with a mental health requirement. Bradshaw has an IQ level often associated with a learning disability and has been diagnosed with schizophrenia. He has been jailed 11 times for persistently breaching a court order after being convicted of exposing himself on the steps of Leeds town hall in 2006. After he was found sleeping rough in London and appearing at the Old Bailey, he was given a hospital order, and cared for in a mental health unit in Manchester between June 2009 and April 2010. On his discharge he went to stay with Liverpool, and stopped taking his medication.

By January 2011, police believe, the conspiracy plot was 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.

Texts to contacts in Manchester found by the police showed Liverpool had tried to source semi-automatic guns, silencer and night-vision kit. 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".

Sentencing Liverpool the judge said it was clear he had been planning to rob someone of what he "hoped would be in excess of £1m". He had chosen Stone as a "random target" because he thought she was wealthy and assumed "she was a friend of the royal family" and would be able to give them that sort of amount.

He said Liverpool had recruited Bradshaw into the scheme. "It may have been … 'a crazy scheme from a crazy person and must be likely to fail' but when you decided to travel from Manchester to Devon you intended to carry it out," he said. He was considering a hybrid sentence for Bradshaw, meaning he would receive a custodial sentence but would most likely serve it in a secure psychiatric unit.

Stone lived among the community where she ran an "open house" to which neighbours were encouraged to pop in. She told the court she did not have a proper lock on her door or set her alarm. "But I do now," she added.

In a statement after the verdicts, she said: "I am relieved the trial is now over and that these men are no longer in a position to cause harm to anyone".