HI Paultownsend
That is perfectly normal, dont worry about it . if you want you can turn down the backlight and brightness a bit, but this may make the viewing picture a to dark for you. During most viewing you should not notice it just as you say with films with black bars at the bottom or sci-fi space movies and otherdarkstuff such as lord of the rings
My £1600 samsung also does this slightly, its why plasma fans don't like lcd/led.
Personally i just find plasmas a bit dark, but its my "own opinion" before the plasma fans get their knives out!!! Ha Ha!!!
Cheers Poolmaster