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A man - living in Dover - arranged to meet a school girl 10 years his junior for sex after befriending her on Facebook.
But he was thwarted by a friend of the teenager who planned to confront 23-year-old Dean Luckhurst at a railway station near Sittingbourne and then report him to the police.
She was commended by the judge for her actions which had prevented Luckhurst from carrying out his plan to have sex.
Luckhurst told a jury he had arranged a rendezvous with the child to "let her down lightly" after discovering her age after they had already had sex.
But a jury at Maidstone Crown Court rejected his claim and convicted him of inciting a child to have sex.
Judge David Griffith-Jones QC said: "I reject his claim, absolutely. He said he was horrified when he discovered she was only 13 but that didn't stop him continuing to arrange for them to meet up again.
"I think he didn't alight from the train in March because he realised he had been rumbled and his claim that he had been horrified, angry and concerned at finding out her true age were preposterous.
"He realised he had been rumbled and his claim that he had been horrified, angry and concerned at finding out her true age were preposterous" - Judge David Griffith-Jones QC
"I watched him giving evidence and I found his presentation chilling."
Luckhurst, who has no fixed address, had met the child at Teynham rail station earlier before walking to a wooded area and after smoking cannabis, had sexual intercourse.
Luckhurst claimed it was only later he realised she was under the legal age of 16 years old.
He denied four child sex charges which were alleged to have happened in February and March this year.
The jury found him not guilty on two charges involving paedophile hunters and couldn't agree on a third charge that he knew she was under age when they first had sex.
The Crown Prosecution Service decided against asking for a re-trial and he has now been formerly acquitted.
Prosecutor Vivian Walters revealed how Luckhurst had contacted the victim via Facebook where he had "groomed her".
Later she spoke to an older friend who knew Luckhurst and the police were alerted.
The judge said that Luckhurst had used the internet to contact girls and was "not concerned about their ages" and as a result he ruled that he posed a danger to girls.
He then passed an eight year extended prison sentence, in which he was jailed for four years and was then ordered to serve another four years on licence when he is released from prison.
Luckhurst will also now be subject to a Sexual Harm Prevention Order and a restraining order preventing him from contacting the victim, who has become withdrawn and depressed as a result of the incident.
Investigating officer, Police Constable Jenny Boyden said: "These were abhorrent crimes committed by Dean Luckhurst, who manipulated a child in order to satisfy his sexual desires.
"I would like to pay tribute to any victim for having the courage to come forward with information that allows us to ensure people like Luckhurst are sent to prison.
"The outcome of this case will hopefully reassure all victims of sexual offences that we will always listen to you and provide you with all the necessary support as we investigate the offences that have been reported to us."
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