Philips 42PFL9664 grey mark in the centre of screen

[Garry123]

I recently noticed that my 42PFL9664 has developed a grey smudge in the centre of the screen, about 3cm square. At first I thought it was a dirty mark. It is only visible when there is a white back ground. Is the back light failing? I have had the set for at least a couple of years.

I have tried increasing the brightness and contrast all the way to max, but it is still visible. Any ideas?

My wife thinks I have sabotaged it .