Detectives hunting the killers of an Edmonton teenager last year want to trace the driver of a silver BMW who may have seen the attack.

Officers believe the driver, who appears in newly released footage, may have crucial information. The CCTV shows 15-year-old Negus McClean and his younger brother being chased by a gang of seven boys on bikes.

Negus, an Edmonton County School pupil, was stabbed to death in Westminster Road on April 10 last year. He was trying to stop his brother’s mobile phone being stolen when he was attacked.

The teenager was beaten with metal poles before being stabbed in the thigh and chest with such force that the blade snapped off in his body.

CCTV shows the two boys being chased by the gang on bikes, before Negus shouted to his brother to run away and confronted them.

Detectives have posted an appeal on Facebook and the Met is offering a £20,000 reward for any information that leads to the arrest and prosecution of those responsible.

Sixteen people aged between 14 and 25 were arrested last year and remain on bail.