Peter47. Years ago Heathrow had a scheduling committee, which included reps from ATC, airlines, airport authority, etc. It was designed to prevent major holding by spreading the schedules. I don't know if it still functions but it didn't work too well as most pax want to travel either morning or evening.. Trying to organise every flight to arrive or depart at a specified time to avoid delays would be just about impossible given the number of variables in the equation. At least twice attempts have been made to partially "computerise" the approach sequence at Heathrow but both failed because the machines could not cope with the variables. It's like saying if we give every commuting car driver a precise time to leave home we can avoid the hold-ups on Battersea Bridge....