I do not wish to post the link "Taking Britain Further" (it may go wrong) but it is readily available thru' many search engines. The diagram on page 25 shows Heathrow EAST & WEST. On page 51 item 8 on the page, is described as "two main passenger terminal and public transport areas". You may note that T5 extends further west to engulf the north south road.
This is what I rely on in describing East & West.