OK so problem fixed but by using the F12/reboot trick in the following link - very odd but it worked.
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Interesting - I didn't know about "action centre" but in there I found "Display" settings which are set to maximum brightness, so it's not that sadly. I didn't see a "back light" setting anywhere -...

My Lenovo Z500 updated to W10 and the display is now dark and hardly readable. If you press the up/down keys for display the little control pops up on the screen - the bar moves up and down but the...