DaveReidUK....are you sure about the runway pair in direction 15/33 being initially designated 16/34? It's new to me and I thought there was visibly a 10 degree slant relative to the original central 16/34 and it was all before the magnetic shift.
The position of Perry Oaks sewage plant continued to dictate the layout of Heathrow, preventing T4 from being built between the runways instead of on Perry Oaks (as T5 eventually was)