Anyway, now that the conservatory is done, I have been able to finish off some of the details prior to moving on to the roof construction phase.
I glued on a block of wood for the chimney base, using a scrap left over from the foundations. It wasn't quite thick enough to fit snugly, so I packed it with two coffee stirrers, then cut them off flush once the glue had dried. The joint still gaped a little thanks to my inability to cut straight, so I applied a little more filler after I took this piccy.
As this was virgin ply, I had to treat it as I had the kit pieces and do the sealing / sanding / gesso / sanding/ coats of paint routine. Then I glued it in place.