Vertex Selection - add to

When in vertex mode for a reasonably dense mesh, I select some vertices with box select and that part works fine.

Then I want to add to the selection so use the Shift key with box select to select some more vertices and that works fine but it turns off previously selected vertices.

Is there a way to stop it turning off previously selected vertices ?

I haven't been able to find a tool or setting that controls this behaviour.

In the "Tool Search" window, find the hotkeys for these tools.

Include Selection
Exclude Selection
Toggle Selection

Got it, it was Ctl+Shift & Alt+Shift is what I should been using 

Exclude Selection Alt+Shift
Include Selection Ctl+Shift
Toggle selection Shift

Working much more productively now, thanks !