Hi jster
I've been pretty busy so I haven't been able to update my components following Howard's very helpful input; hopefully sometime soon!
I initially tried to create a component that would cut the two wall planes in one go but as far as I can tell this is just not possible. Which is why I created the 'Front' and 'Back' door components; not ideal but better, I think, than forming an opening and placing a simple door component.
I had to orientate both the 'Front' and 'Back' components around the same 'lengthwise' start point so that, once placed, so long as you change the size of the opening on both components the opening stays aligned. The specific sizes were to suit 'standard' British door sizes but I can see this won't suit everyone so I'm planning a suite of components from very simple openings (with variable input) to more complex specific 'real world' door sizes and types. Not sure when I'll get around to uploading all these - soon I hope!
Hope this helps, S.