The north side apron was the original for Heathrow when it first opened. It remained in use as London Airport ( North) until the prefab buildings, which consituted the long haul terminal were replaced by Terminal 3 in 1961/2. It covered roughly the site from just east of the tunnel , through the current Visitors Centre and on into the taxi park.
The original plan for LHR circa 1948 shows no less than 9 runways, of which 8 would probably have been eventually operational, with 3 in a triangle between the A4 and where the M4 now is. The villages of Sipson and Harmondsworth were both marked for demolition until released from this threat circa 1953 when the government reckoned that the airlines would never need such a large facility. That was the last of the vision and we have been living with the results,- and accountants,- ever since.