Clearly, this is a backlight problem.
What I find odd about it, is that just turning the screen off and on again will fix the problem for a while.
I opened it up. As far as I can see, there isn't a separate connection for each light source, and anyway, thumping the device as hard as I dare doesn't make any difference.
There also doesn't appear to be a separate a inverter (would have surprised me if there had been).
Probably headed for the tip, but any thoughts?
Sylvia.