Ok, so here's the culprit:
And here's the problem:
Looks like a crack in the scribble strip screen, but it's a cat hair inside Grrrr!
I've taken the back cover off and blasted it with air, but I can't dislodge it.
Before I start randomly taking screws out, does anybody know which ones I
need to remove to ease the board away enough to give it another blast with
compressed air to remove it?
... and yes, I will take full responsibility if I break it, so no litigation worries :-)
Any help from you wonderful people gratefully accepted.