To get behind the surface appearances here we need to, well, get behind the surface. This is the fireplace from the previous post, viewed from inside. This shot (sorry about the quality) looks directly up towards the chimney pot three stories above. Note the continuous brown support about four brick courses up, and the way the shaft changes from vertical to a slight angle at that level. This change is not visible outside on the room wall, so the chimney breast thickens at that point. The four courses below that are held up by a rusty L shaped metal cross support, a tiny part of which is visible bottom right.
This shot shows that the long brown support in the shot above is actually a very thick minimally shaped wooden beam. At one end it is nailed with a very thick old fashioned nail to a metal bar which crosses the width of the chimney and is fixed to the back wall. Just visible below it on the right of the shot is another large nail fixed in the side wall. both the bar and the separate nail stand proud of the wall by an inch or two. The next and final post on this puzzle will add a couple of other bits of evidence and draw the interpretation together.