They do not alternate on easterlies, and have not since the mid 50’s when jets first arrived, its the Cranford agreement, withdrawn in 2010.
HAL submitted plans to Hillingdon council who refused, HAL appealed, they were granted. HAL reneged on its own application and deferred to R3 completion saving themselves £millions. Therefore 18hrs continuous noise on the runways, and you trust them to build a new runway taking account of public interest?