LCD Monitor Creeping Darkness

I have just bought a refurbished LCD Monitor (Hanns-G HX193) which was excellent for the first few minutes (!) then the display changed dramatically. It is still clear, but much less bright; and the odd part is that the degree of darkness increases from top right (near full brightness) to bottom left (very dark, almost impossible to read text). It looks as if Mordor is invading from bottom left!

The good news is that the vendor (the excellent Novatech) has apologised - it was obviously OK when they tested it before sale, and when I first switched it on - and have also given me a complete refund. They don't even want the monitor back. The bad news is that, while I would like to fix it if possible, I don't know where to start. Could this symptom be caused by some kind of partial backlight failure - is such a thing even possible? Or is it likely to be something more complex - am I wasting my time with what could be an expensive repair?.