Solved
Centreline in Drawings
I have a circle and would like to create a centreline through it, so that I can use it as a reference when doing dimensioning. How would I do this?
With a drawing open on screen...
Select the Annotate tab
Click Show Model Annotations.
Select the view you want centre lines on.
In the Show Modle Annotations dialog, click the last tab on the right then click 
to add all centre lines to the selected view.
Select and delete the ones you don't want and extend/shorten the centre lines you want to keep.