From my dim and distant memory of such things Dover's on Westerlies have a standard 3 min between them due to the raking left turn after departure. There are so many interpretations of this turn that 2 minutes can sometimes put consecutive Dovers embarrasingly close together if the second one is a bit quicker and a bit tighter in the turn. The classic used to be an A340 followed by a 747. The A340 gets airborne and used to be told "no atc speed restriction" at which point they would gleefully come back to 210kts and be eaten alive by the Jumbo 2 minutes (that is a tower 2 minutes which in my experience is somewhere between 61 and 80 seconds) behind. Anything longer than 3 minutes is indeed to due flow regulation which could be due to a number of things. The most likely cause is simply too much traffic forecast in the DVR sector at the same time but it could be due to Weather or there is also an enormous danger area owned by the belgians (they use it for exercises in their attack gliders) I think which can have some severe restrictions on the use of the old greenway.