Detectives who caught a would-be armed robber have revealed that a Marks and Spencer shopping bag and a packet of sliced ham were key clues in their investigation.Benjamin Barton, 18, of Clovelly Road in Bideford, appeared at Exeter Crown Court on Thursday, where he admitted attempting an armed robbery at Coopers Jewellery in Barnstaple High Street in May this year.Barton was remanded in custody and will be sentenced on August 23, but following his conviction, Barnstaple detectives have revealed how he was caught.Shots firesBarton wore a latex head mask he had purchased online and armed himself with a pistol filled with blanks on the afternoon of May 28. But jewellery shop owner David Bruce fought back with a baseball bat, knocking the magazine from the gun and causing Barton to flee empty handed, leaving behind his plastic shopping bag. Detectives have now revealed DNA found on the bag, the magazine and the remaining bullets, was the first step in finding the culprit.DNA discoveredDetective Inspector Praveen Naidoo, senior investigating officer, told the Gazette: We got a full DNA profile but there were no matches on our system. It was surprising because it is very unusual, although not unheard of, for a person to go from a law abiding citizen to attempting an armed robbery. Faced with a nameless, faceless suspect, Detective Sergeant Simon Butler said the next step was to investigate the items at the scene. He said the bag was identified as a Marks and Spencer bag for life, the design of which had only been on sale for a few months.Odd purchaseDS Butler said: We worked with Louise Coley, finance and operations manager at the Barnstaple M&S store, to find any purchases of a bag for life made in the last two weeks. Louise had already flagged up a purchase she thought seemed odd, of a bag for life and a packet of £2.50 ham; usually such bags were bought with a larger quantity of shopping. They matched the purchase to instore CCTV of the customer but police still had no idea who he was.CCTV appealOfficers first released the image to local schools, eliminating several people from their enquiries. Still without any leads, the CCTV was put out in the media. As a result, several days later, Barton turned up at Barnstaple police station, claiming he was the person in the photograph but knew nothing of the robbery.Barton arrestedDS Butler said: We spoke to him and took his DNA as standard, and asked why he had come all the way from Bideford to buy some ham. It seemed a little strange at the time but both the officer in charge, DC Simon Scotthorne, and I did not expect he would be a match. But on June 30, forensics confirmed Barton was a match for the DNA recovered from the scene and he was arrested and charged. DI Naidoo added: Its never nice to have an offence like this on your books, but it was a fantastic bit of policing work by the team.