The lobby I may use as a stand along privacy (even signalman have to meet the call of nature!).
There is also a nice drawing of the box to 2mm scale in the book.
I had wondered what sort of chimney the box had and the photo/drawing has answered the question, brick plasticard and 3D chimney pot should sort that out.
For now I have test fitted the roof, as although the back of the box will face the viewing side, it cries out for interior detail! Once everything is in place I shall spray undercoat the box after washing it in iso alcohol as suggested to remove the moulding solution.