We seem to have made "yet another" significant discovery.  We have what appears to be the 'Fire Pit' for the system which would have heated the damp air in the Wheel Pit Cellar to drive it up the hollow centre(s) of the spiral staircase(s).  Any chance of you coming down over the weekend to help in the "informed guess" process? I would really appreciate it if either or both of you can. A couple of pictures attached.

The fire pit.