Once again thank you for your reply. Unfortunately I appear to be still looking at this all wrong. I have studied the manual on creating openings and all it says to do is create a 2D and 3D block, assign them to the Opening properties, make sure Anchor to Wall is selected and then drag the block onto the wall. Actually, taking it literally, I am unable to 'drag' the block because as soon as I click on the Openings icon the block automatically appears and although it follows the layout of the wall with the mouse it just wont place itself when I click (as you will see from my example drawing).
Anyway I appreciate the time you have given me and it is obvious I have a lot to learn about reference points - hopefully one day it will all make sense - thanks again