Yes BIPLAB, you're right, and I have now got it to work.
For some reason, my walls which are drawn on a class Wall-Ext-Frame (set to use at creation), and type Unstyled, were in fact not behaving as I expected. If I select the wall, change it to any other class, then change back to Wall-Ext-Frame I get the correct result.
Can you explain why the wall, whose class was set to 'use at creation', was in fact not using at creation?
Thanks in advance.
Mat