I realise this is a total noob question but...
I can't seem to find a way of editing objects outside of the viewer. I'm working on a landscape that needs to stretch a good way horizontally but can't seem to adapt the comp viewer to accommodate.
What silly thing am I doing wrong?
You can use the other views if you are using 3D layers. Just select 2 or 4 views in the Composition Panel. If you are using 2D layers then you are stuck with what is in the Active Camera view. More information and a screenshot would help us point you to the right workflow. I do this kind of thing all the time using 3D layers.