The HS-125-400B aircraft was parked on the international departures apron at Lanseria Airport, South Africa ready for a flight to Blantyre, Malawi. The pilot assumed that the engineer had pressurised the hydraulic accumulator and therefore did not inspect it himself. When the passengers arrived, the pilot got in the aircraft and began the internal pre-start checks. The co-pilot removed the chocks after boarding the passengers, closed the door and then started to brief the passengers. At that moment the pilot noticed that the aircraft was rolling forward. Attempts to stop the aircraft were unsuccessful, the aircraft rolled forward, narrowly missed a hangar and a parked Bell 206L LongRanger helicopter next to the hangar, crossed the taxiway and finally stopped when entering a ditch between the taxiway and the runway. Probable Cause: “The hydraulic system was not pressurised during the pre-flight inspection of the aircraft. The chocks were therefore removed without adequate hydraulic system pressure available.”