Update: seem to have cracked it - I un-installed SweetHome 3d and ensured all folders were removed, then deleted the Sweet Home 3D Folder found at: C:\Users\{me}\AppData\Roaming
Reinstalled Sweet Home 3D 3.4 and it's launched first time.

Hope others find this useful as it was driving me nuts!
Thanks again for providing an exellent piece of software.