PLEASE READ IF YOU ARE USING THESE DIMMER SWITCHES!!!
Before I went to work this morning, I switched off all the ground floor lights at the fuse box, just be on the safe side. It was bloody lucky that I did.
When I got home from work (about an hour ago), I switched on the ground floor lights to have a play, and as expected, the lounge light was still jammed on 100% (as I mentioned in the edit in my original post). I then sat down in front of my laptop with the aim of removing the light from the system and trying to add it again, not that I had much hope that this would work. About 15 minutes later, my girlfriend came back from the shops, and the first thing she said when she got in the door was that she could smell burning plastic. The dimmer switch was hot to the touch.
This is what I found after I had removed it. As you can see the label has melted away, but you can also see that the white casing itself has also started to melt.
So for Christ's sake, if you suspect a problem with ANY electrical device, DO NOT continue using it and GET IT REPLACED. Sorry to be melodramatic, but I just want people to be aware.