Thanks for the replies everyone.
I had a look yesterday - I'm pretty sure it's LED, as I'm assuming a plasma would just look like a blank screen when it's turned off, wheeras this you can definitely see individual cells on it.
I had a quick look at the connectors, especially for the ribbon cable that connects to the screen. I wasn't sure what I was seeing to being with. As i tried to gently pull the connector, the two plastic 'wings' at the side were moving around. I thought something was broken at first, but then I realised they were meant to clip the connector and hold it in position.
'AHA!' I thought. 'Bingo!' These weren't clipped in position properly when I first looked at it, so I thought that might have been the problem. So I took the connector off, put it back in position, making sure the clips were in the locked position, turned the game back on and...... No change. hehe.
Oh well. It looks like it will have to be a replacement, if it either gets worse or bothers me too much (I don;t know that it will bother me too much)
I don't have the skills or tools to go soldering in a new part of the display.