So having added the firebox door yesterday, today I added the regulator handle and the two gauge glasses. In the process I managed to remove the firebox door and the boiler wrapper, so the entire backhead has now been rebuilt.
It's not quite as neat as it was before and the lesson is that I need to buy some low melt solder for adding details without unsoldering the main parts. While it may not be pretty I'm still happy with it as it is still performing the job I'm building it for... to give an impression of a cab interior when glimpsed through the door.
I don't know about you but I think that does the job adequately enough.