I am much indebted to Jack Clarke for providing some further information to the 'valve on the chimney' question. http://unnycoombelala.blogspot.com/2011/11/question.html
Young Jack suggests that is a displacement lubricator 'I am 98% sure,' and has led me to the following illustrations to back this up. The following is a scrap from a GA drawing showing a link from the regulator in the cab to a lubricator mounted on the chimney. Steam is forced in, where it condenses to water, forcing oil, which now floats on top, to run down a pipe to the cylinder. The reasons for the chimney mounting are purely gravity linked. Further details of Mr. Ramsbottom and his injector can be found here:
I am rather embarrassed in that like so many long-term modellers, there is so much I don't know about the workings of what I represent in miniature.