Trials took place in 2014 to test new operational and technical procedures. The trials resulted in a lot of planes flying over certain areas and in a concentrated way. Following complaints from some people in the communities affected that the flight paths had not gone back to their pre-trial patterns, Heathrow paid for work to test this out. The reports found that flight paths had returned to their pretrial routes. The steering group for the work was made up of representatives from the affected communities. The steering group set the brief for the work and appointed the consultants. The areas the report covered were Sunninghill, Bracknell & Wokingham, Englefield Green, Lightwater and Teddington & Twickenham. Although things went back to normal after the trials, the reportdid identify some changes that had taken place incrementally over the 10 years or so between 2006 and 2016. Departures had become more concentrated along the centre-lines of the Noise Preferential Routes. On some routes the height of aircraft increased, but on others the planes were flying lower. It also identified a gradual increase in the number of aircraft on most routes, with the Teddington/Twickenham route seeing a particularly annoying increase of large, intercontinental traffic in the mid/late evening.
You can find the reports here:https://www.heathrow.com/noise/heathrow-community-noise-forum/flight-analysis .