Thanks for the video. This really helped. I would like to mention though that the video outcome does help if you create a 2nd floor first, and apply the routine to a 2nd floor. I tried the video's method first, but found out that the dormer outside wall belonged to the first floor wall. This meant that when placing windows on the dormer outside wall, the windows actually ended up being on the first floor, meaning I had to changed the 'from floor' specs of them to push them up to the dormer level, but then when it came to changing the interior material of the walls, as it was one wall from the 1st floor, it did not work. I looked at pony walls, but decided to redo the whole lot with a 2nd floor before the above tutorial.
All sorted now, so thanks once again for your assistance.