Well, I want to know how a particular docking feature works.

I have a 24" monitor I can put in portrait mode, and so I'd like to be able to have one tab down the left hand side, which I can divide vertically into 2 or 3 tabs.

However, try as I might, I can not get a tab to dock anywhere other than to the left or right of an existing tab, or as another tab in the docking window.

I noticed that somehow my 'debug' window configuration had got two tabs split vertically the way I wanted, but I've no idea how that happened, and I tried playing with the windows to see if I could work it out. However in the process I just lost the vertical split from my debug config too, and I can't get it back

Is there some magic mouse position I need to move the cursor to while dragging a tab/dock window in order to make the tabs split the space vertically and sit one above the other? Or is there some keyboard shortcut?

I hope I've made that clear enough

it's not hard:
undock the windows you need to be grouped
move main window away (temporarily)
dock the windows together starting with any one of them
dock the resulted window back to the main window

Aha! That works - many thanks