I have another couple of shots of progress on the boiler. The first is the bottom tube plate and the fire-tube after soldering and pickling.
There is a nice fillet of solder at all joints which is what I was aiming for. The citric acid pickle also cleans all the gunk off very effectively such that all that is required is a rub with a scotch brite or wire wool under running water once the job comes out of the bath.
Here is another shot showing the top tube plate with all the bushes soldered in.
Here is the underside of the top tube plate: