I actually found a work-around for it that appears to work. If when I create the tall wall (the one at the 45 degree angle) I add a very small 45 degree corner wall segment at the end of it, this then gives me my perpendicular wall joint to the shorter wall. Not sure why I didn't think of this before, but as I said I am fairly new to this.
However, I think Nemetschek needs to think about giving more flexibility to adjusting wall join angles to make this a simpler task. I did have to go back and recreate both walls to get this to work the way I wanted. I had six windows which I had to re-insert.
Thanks again for your suggestions.