Paul - you got me thinking!! So I went and looked at every picture I could find of this class and compared them with the chimney which came with my original Acorn kit. A 'niggle' I had soon proved fact. The Acorn kit chimney, which is white metal, gets wider the higher up it goes. This is wrong! The chimneys on this class were 'stove pipe' type according to all the pictures and my drawing, and the same diameter from top to bottom apart from the flange around the top and the 'skirt' at the bottom where it joins the smokebox.
Scorpio do a lost wax chimney intended for their upgraded Acorn kit of the 77xxx, which is the tender version of the 82xxx, and I had one of these. It checks out measurement-wise and looks perfect, so it is now on my model!
Brian L.