Thanks, starting to understand a bit more, but knew there was something about 15 minutes from the P time and a re-file was needed.
Sabenaboy:
No, the AIP chapter that I mentioned is about flight plan filing and this rule is apparently applied to all flights, even if they didn't receive a slot
All flightplans enter the system and still show up on flow control charts, so depending on what times are entered for the departure/FIR boundaries depends on whether you get a slot or not. If you don't get a slot then it means that based on filed times a slot isn't needed, all a slot does is limits the30 mins gap to a 15 mins gap so we can be slightly more accurate in when you're going to arrive. At Swanwick every supervisor position has flow charts that show block graphs in 15 minute segments (although not very accurate) and when "peaks" start appearing in a 15 minute gap then they look to whether the sector can be split and if not, apply a form of flow control to regulate the movements. This ranges from basic flow (slots), Minimum Departure Intervals (MDI's) applied to a particular airfield thats going to deliver most of the a/c whereby rather than one every 2 mins on a route, it'll be increased to delay some traffic. Also level caps can be applied to vertically split sectors, in London to protect the London Upper Sectors (S1/S2) some a/c will be hand picked to descend below FL310 and transit through the Middle sectors (S25/S26), normally those going to Manchester/Liverpool from the south.
Have had another thought - could it be to do with the Spanish ATC system? If it has a pending time entered for an a/c and then gets airbourne early then the sector may not be expecting it and therefore have no details, is there anyone from Spain who could confirm or deny this?