As I typed this post I’ve got it working – this screenshot shows that the composition appears out of shot. I’ve no idea why this happened, maybe the size of my compositions mucked with the alignment? At any rate, thanks for the replies. I’ll leave the rest of the message here, just on the off chance it helps any other idiots..

—
Okay, for the sake of clarity: Comp 1 is called “TheEdge”, Comp 2 “MG_copy”.
Walter, Collapse Transformations aren’t toggled on, but I’m really glad you mentioned the switch, I’d not know how to retain the 3D elements of a precomp before. I’ve turned it on.
Ross, the 3D layer switch is on for the precomp.
The first screenshot show how the layers within MG_copy should look – they’re visible in all cameras except Active Camera (does that make a difference?) – and the second screenshot shows how MG_copy appears while it sits in TheEdge. Just a bounding box.

