Losing Selections when using mouse

[Fad]

When using NC mode I can use the "Insert" key and select several items automatically...the focus moves onto the next file....

then I can use the Up & Down keys to choose any other file and press the Space bar to select it.

When I then try to select an item with the mouse and highlight it with the space bar, all the other previously selected items become unselected.

Is there any way to prevent this from happening, I can`t see it in the options ?

In TurboNavigator I can do this, and all previously selected items remain selected even if I choose another with the mouse.

Edit:
I have just realised this works OK when Extended Selection using Shift & Ctrl is turned OFF....but not when it is on.