andrew hennells posted message on facebook admitting his plans to rob a tesco man who boasted about plans for an armed robbery on facebook minutes .
over ' at 7.25 pm , the 31-year-old , of gaywood , king 's lynn , admitted robbery and carrying a knife as an offence .
hennells posted this message only 15 minutes before threatening staff at tesco 's in king 's lynn .
last month , norfolk police were alerted by a member of the public to a man brandishing a knife demanding cash from staff working .