Thanks Conrad,
Hovering at the top of the screen doesn't get the tool bar back even though I have tried the mouse cursor in various positions. I note that the "Pin" gadget is activated all the time (when the tool bar is present after a restart) and I can't toogle that off.
The F12 and other keyboard shortcuts that you gave at least give me a work around saving me from having to forceably shutdown my laptop.
I also have the Viewer installed on a Intel Compute Stick, also running W10s and this doesn't have any issue with the tool bar disappearing. The only issue with the Compute Stick is that it is way to slow for remote access.