Admin nav-menu levels

Hi,
I’ve had a good search around and can’t see where anyone has queried this topic previously.
The Nav Menu building structure is fantastic and I have used it a lot. The problem I now have is that my latest site has a large amount of categories and pages to put into the menu. No problem so far but when I get to put in the level 2 and level 3 items it get quite awkward to manipulate the admin window. It becomes an extremely long vertical run which I have to continuously run up and down adding and editing the content.
My point:- is there currently a facility in the back-end admin area to bring all the level 3’s under their level 2 item so that they are only visible when the level 2 is opened like a vertical concertina menu? This would also be useful for the level 2’s under their respective level 1.
Or is there a plugin that is currently available? I’ve looked hard but without success…
Fred