A NEW Mercedes-Benz 600 SEL worth $2.3 million was last night hijacked in Sha Tin and its owner and chauffeur threatened at gunpoint. Police said the 26-year-old chauffeur was driving his 59-year-old employer home along Taipo Road when their car was intercepted by a BMW with three men and a woman on board shortly after 10 pm. Two of the men walked to the Mercedes and threatened the victims with a handgun. The two victims were pushed into the back seat and the car was driven to the Shek Lei Pui reservoir. The pair were then tied up with nylon string and pushed out of the vehicle, which sped away. The victims, who were not injured, freed themselves and called police from an emergency telephone.