Anyway, having got the second x200 to replace a c50 with growing backlighting, my stomach took a sudden plunge to the floor when I saw what appeared to be a backlighting problem with the brand new monitor!
When installing Mint Linux KDE 17 when I moved the mouse pointer to the top left hand corner that corner blurred, producing a bight blue 'wipe out' very similar to backlighting. The problem did not occur anywhere else on the screen, no matter where I moved the mouse pointer.
The problem also occurred when I tried Mint 17.1, Kubuntu and PCLinuxOS!
However, it did not occur on my four core x200 laptop.
This strongly suggested a hardware fault - to much stress after masses of hardware troubles of late.
Anyway, the light came on when I failed to recreate the error using Cinnamon.
It turned out that screen wipe out was caused by desktop effects - almost certainly transparency.
I always turn desktop effects off after installing KDE but, of course, they are on by default and during installation. When I turned off desktop effect, the problem went away. When I turned effects back on on the other x200 I got the same wipe out.
I suspect this is a peculiar combination of x200 tech and KDE but, should anyone else encounter this… just turn off desktop effects. (They only eat up processor power anyway.)
Oh, and if users are having trouble with the over sensitive touchpad of later x200's - install the 4.0 kernel. With Ubuntu binary repositories it is easy!
