Unfortunately, ALT-F7 does not work with child windows within C::B. Neither does holding down the ALT key and dragging.
I have, however, solved the problem! Right-clicking on the top panel and selecting "Auto hide" means the panel slides up out of the way allowing me to grab the title bars of the C::B debug windows and move them somewhere sensible.
(I suspect the cause of the problem lies in the source code: Probably the default initial positions of all the windows other than the Watch window are at (0,0), thus under-lapping the Gnome panel. Not quite a bug but a highly undesirable feature - could it be changed?
P.