Thanks Scrotchidson, always good to get things from someone who is most knowledgeable about ATCO !
It's when the smaller ones depart they get in the way whilst they climb through the LTMA. It affects a couple of airports, some more than others, and until the airspace is "upgraded" to suit the year we're in things will remain the same unfortunately.
I remember a curtain retired Luton ATCO used to say about this, Northolt & City come to mind, I don't envy your 3D puzzle!