The Cranford Agreement
SpanishFlea - in a nutshell. Cranford is a 'village' at the Eastern end of 09L/27R just to the North of the centreline. Many years ago in the times of wizards and dragons the natives decided they didn't want all of those noisy Comets/707s/Tridents/whatever flying low over their houses so The Cranford Agreement was born restricting/preventing departures off 09L. At the Eastern end of 09R/27L there's only a tube station, hangars, a pub and some cows and horses so this was a sensible idea.
What was nice though was during Concorde's last year of operation part of 09R/27L was being resurfaced and there was a bit of a 'join' between surfaces in the work in progress. Following the recent Paris crash it was deemed sensible to limit Easterly Concorde take-offs to 09L much to the delight of the good people of Cranford/Heston.
sB