Can't seen primitive options when adding a new mesh When I add a new mesh, I don't see any primitive options. Ie. being able to adjust the number of vertices etc. 

If I create a brand new file then the option to adjust the core mesh primitives is visible in Properties panel. So it appears to be linked to the file I'm working on. 

Driving me nuts.
I'd somehow hidden the 'Operator' panel.

For reference for anyone else:

Open the Tool Shelf 'View>Tool Shelf' then click the tiny plus icon just above the 'View Select Add' menu. It should be at the very bottom of the Tool Shelf Panel.