My late father was an early arrival on the scene. He was a serving Met Police dog handler, and was on duty in plain clothes patrol, along with another dog handler, in the grassed area between KGV and Wraysbury reservoirs as part of the security patrols in place at the time of the Army anti-terrorist patrols when intel indicated the IRA had acquired some RPG's and might try and take down an airliner on take off or landing at Heathrow. There was a fair number of Army personal around the airport at that time.
The aircraft passed almost over my Dad's head about 15-20secs before it impacted, and he guessed it had crashed beyond by the instant silence, and he and the other handler, made their way back to where they had parked their cars to put the dogs away by which time sirens etc., drew them to the direction of the scene. It was sadly the 2nd air crash scene he had to attend during his 30 years service, the first being the crash of the Viking into houses of Kelvin Gardens in 1958.