From what I understand arrivals to Gatwick, Heathrow and other major European destinations will have been subject to a slot time for departure, using a centralised computer system based in Brussels that determines capacity for a given piece of airspace or concrete and does not permit more than a set number of aircraft to arrive in a given hour. The delays are absorbed on the ground or schedules are adjusted accordingly preventing excessive airborne delays.
This kind of system is planned for Aus and has been patchily attempted, but has not been critical until recently as airborne delays into our major ports haven't been long enough to make the accountants cry or overload the ATC system. We are probably approaching that point now. Note - this will not increase the arrival rate, it just means the top of the peaks will be forcibly shoveled into the troughs with the scheduling complications to be dealt with by the companies. Its not all beer and skittles - In europe if you miss your ramp time well thats tough titty and back you taxi until another slot is available.
My experience is that too much less than 90 seconds between arrivals and number 2 will be going around. Something more than that will be required if tower is to have an opportunity to depart one occasionally. This places a practical upper limit on how many aircraft can arrive within an hour without someone having to wait, you can do it on the ground or in the air, but unless there is more concrete the delay will have to be absorbed somewhere.
WWT