text: Andrew Hennells, pictured, posted a message on Facebook admitting his plans to rob a Tesco supermarket 15 minutes before holding up staff . A man who boasted about plans for an armed robbery on Facebook minutes before he threatened staff in Tesco has been jailed for four years. Andrew Hennells posted details of his plan to the social networking site - stating he was 'Doing. Tesco. Over' at 7.25pm on February 13. A quarter of an hour later, Norfolk Police were alerted by a member of the public to a man brandishing a knife demanding cash from staff working at the supermarket giant's King's Lynn branch. Hennells fled the scene before police arrived, but he was quickly arrested in a nearby pub with £410 in stolen cash. Norfolk Crown Court heard that officers also recovered a knife. Last month, the 31-year-old, of Gaywood, King's Lynn,admitted robbery and carrying a knife as an offence. Sentencing him to four years in prison for armed robbery today, judge Anthony Bate said Hennells posed a 'high risk of serious harm to the public'. The court heard how armed police were deployed after the alarm was raised. Hennells had escaped after stealing a car belonging to a retired couple who had stopped to use a cashpoint. But officers tracked him down in a nearby pub, finding him carrying a knife and £410 in stolen cash. Another count of robbery relating to the car-jacking was left to lie on file, Norfolk Police said. Outside court, Sgt Pete Jessop, from King's Lynn CID, said that the Facebook confession made it easier to prove the case. Hennells posted this message only 15 minutes before threatening staff at Tesco's in King's Lynn . A member of the public alerted police to a man threatening a member of staff in the store (pictured) Hennells, 31, was then found by police in a nearby pub, with £410 in cash on his person . Speaking after the first hearing, Sgt Jessop had said: 'We have not got to the bottom of why he posted the pictures and words on Facebook, or why he did the job in the first place. 'It was bizarre and an unusual set of circumstances. 'While the message and pictures on Facebook did not solve the case, it did confirm what we already knew. Hennells held up staff at the Tesco store in King's Lynn, pictured on Friday, February 13 . 'But the thing that made it easier and quicker for us to solve the case was the assistance from the public. 'However, the quickness does not take away from its seriousness or how traumatic it was for the victims of the robbery.' Hennells made a series of other posts to Facebook in the hours before the robbery, including a picture of a large knife, and sharing a Bare Knuckles Gypsies' 'The hardest man in Ireland' video.
target: Andrew Hennells threatened staff in Tesco with a knife of February 13 .
  Fifteen minutes before the raid he posted details of his plan on Facebook .
  Norfolk Crown Court judge deems 31-year-old 'high risk' to the public .