Indeed, I'm ever grateful for the multitude of responses and unending advice. :J
Aye, I've come a cropper of the black sludge before, well at least my carpet has. And it wasn't even me doing it then... the plumber we got was a bit of a lummox and when he was taking an old radiator down from the attic spilt the sludge remnants on the stairs carpet. :C
I've taken rads off the wall for decorating behind, but have always replaced the existing one, so with fitting a new one I want to get a match as close as possible 'cos I don't want to have to bend or shift the pipes any more than I have to. Lots of towels and old paint trays are your best friend when taking them off.