Yes it is a Belfast (or Butler's) sink. I'm not sure what the difference is. It's pretty old.
I have opened the flap in the fireplace and removed, pro tem, the firescreen, and I have to say that the surface does seem to be drying out and I'm not getting any more mould, but people are being very careful while using it.
The house tilts a bit to the left, and unfortunately the sink is on the left-hand side of the kitchen, so water does accumulate at the back unless one is very careful not to splash, and it doesn't run off the draining board as fast as it might if everything were completely level. Therefore I have 3 vulnerable areas - the back where it joins the splashback, because of the age and angle of the house, round the tap, and under the drying rack.
I think I'm getting to the point where I can start thinking about sanding it.