@Frazc
Sorry, but with no offense, it is hard for me to believe you.
The described symptoms correspond to 3.1.1 version.
I believe you have download 4.0 version but, from different reasons it is not installed.
So, PLEASE take some screenshots and put it here:
1. Go to Help->About menu and show me whats there (SH3D version, java etc).
2. Go to Help->About->Library... and, on Plug-in section, scroll to GenerateRoof.
If you have Sweet Home 3D version 7.1 and GenerateRoof-4.0
please send me a test file with your issues.
All this could help me (us, the GRT) to fix possible bugs.
Thanks in advance for your patience.