It appears to me that the brick is in the former kitchen passage.
We can see a sloping mark on the wall to the left of the brick and this is
the burn mark where the stairs were. To the right of the brick is what
would have been the way to the cellar stairs.
To be clear I have chopped away at Mr Glanville's plan and I have added the window, there is one on the stairs (It was broken by a stone on March 10th 1931).
The brick appears to be hovering just level where some writing 'MA' appeared. Apparently from M.H.H. a good deal of phenomena appeared around here.