Problem was caused by Qt::ApplicationModal being used instead of Qt::WindowModal
when displaying the settings screen from any of the Indirect interfaces

Had to also set up parents on Qt windows in order for Qt::WindowModal to work:
a) set up parent on the Indirect settings Qt widget when created from any of the
Indirect interface screens
b) set up parent on the C++ GUIs created from Workbench. All the indirect interfaces
are of this type + one or two others eg Direct, ALF View, Indirect Settings when
accessed from Indirect menu rather than via one of the interfaces