After all my messing around with cardboard to produce a chimney for Flockbrugh Fisheries, I solved the problem with a simple action - I handed the job to my Dad. After all, I had enough to do on the train set and it would be a good way to keep his aged brain ticking over.
Of course, pretty much the first version he made was right. All I had to do was cover the card shell with plasticard and paint it. I did take the opportunity to build the top of the stack (is there a technical name for this ?) with multiple layers to make it look a bit fancy. My study of photos showed that factory owners liked to show off up there - presumably because everyone could see where the money had been spent.
Anyway, after a little ground clearance and a touch of trimming at the back, it's in place hiding the gap in the sky where the baseboards meet. I didn't do a great job of matching the brick colours but that can be sorted eventually. In the meantime it's just a more modern addition to the factory to replace the one that nice Mr Dibnah modified.