One mainly just uses one's common, Nigel. The space between two parallel runways, I am sure you would agree, is not a good place to put a big building for your plane to crash into. No more so a bloody great hole in the ground for it to fall into.
Equally, don't put things in potential overrun areas that don't need to be there, e.g. sunken roads, ditches, berms, concrete lighting bases etc. which were only put there because it didn't occur to them a plane could hit them. The Canadian TSB recommended 300M clear flat areas around runways after Toronto, roughly in line with the NTSB's rec of 1000ft. Good enough for me, since you asked. That could well have saved this lot.
They didn't "put" a "bloody great hole" there... it was already there
So what "clear flat area" do you think MAD has established? My quick measurements show ~250m in the area this aircraft left... and for much of the MAD 36L/R construction, somewhat more (and some less). What do you measure it as to that fence? If it is ~250m, and you wanted 300m, given those tyre tracks I reckon the result would have been the same?
LHR fails to meet the 300m in quite a few areas? As I am sure do may others...
NoD