Crikey being called out! As it happens I do fly helicopters and fixed wing. As to how/why 100ft... well in the end its going to be something you either choose to believe or not really, but if you are not familiar with the heath then you can look at google maps.
Find the grass area between parliment hill running track and highgate ponds... the ground there slope downward from north to south but the trees you see there are around give a reasonable point of reference...
Panoramio - Photos of the World
Edited to add photo for ref