@chrisjk said:
You can constrain the movement in editing mode. On the Mac, click Alt (I think it's probably Ctrl in Windows) then the black box turns into an outline, hit the up arrow as you would with the Move tool and Hey Presto you are locked in the Z direction. Fredo links to a tutorial here http://www.aecbytes.com/tipsandtricks/2009/issue42-sketchup.html that explains that plus other stuff.
Aha! This is exactly what I was after... Pressing the arrow keys to constrain direction plane (i.e. up key=blue etc.) is actually a pretty easy way of doing it, I just wasn't aware of this function. This makes it a lot easier to use. Thanks for the tip!