best suggestion mate is to eliminate things from the equation- put the rear seats down and see if it still does it; take out tools from boot (under carpet) see if it still does it.
Keep doing it until it stops - then you know whats causing it. There are so many potential rattles in the leon (and most cars) its better to find out what it is, then go from there.
usual rattle are rear seats, suspension etc. Good luck in finding it - I have a similar rattle but I know where it is.