With your laptop turned ON, remove the charger lead (if it's plugged in). Then, turn it over and remove the battery. Wait a minute or so, restore the power and try again.
I turned it on, turned it over to look for the battery, left it like that for a few minutes while I contemplated the idea of getting a screwdriver to it and when I turned it back up the right way it asked for my fingerprint and touchscreen restored!
Thanks You Chris, I'm delighted and we have a new solution from now on. Switch it off and switch it on again, is now superceded by Turn it over and turn it back up again!