It would have been from the direction of the airport buildings, round that long looping bend on your map and turning back 180 at a point level with the north side of the school field, about the base of the "T" in Heston, where the gravel pit/landfill began at that time. IIRC there was a large earth bank at the end of the track between it and the water of the gravel pit (which was rather convenient to stop cars taking an early bath but was just top soil from when the pit was dug out rather than a deliberate safety barrier)