In regards to the notes from Iris Owen’s personal copy of MMH.
About half way down this page are two pictures showing the familiar northeast corner of the courtyard with the famous bell. One of these pictures show what appears to be the main stairwell window bricked up. The reference for this picture is as follows: “7. Page 26. Referring to courtyard photo, facing page. Note: X This landing window was later bricked up but nobody mentions it. See Plate XV, EBR. Note: No. It's the passage wall showing through after the fire. [First observation seems most correct.]”
I remembered reading from the site copy of Fifteen Months in the Most Haunted House in England edited by Vincent O'Neil. Lionel Foyster writes about a large hall window that had been broken by a “self-propelled” stone. I was able to find this reference in the last paragraph of his Diary of Occurrences. “On the Tuesday evening a stone was thrown from inside which broke a large window in the hall; this is the only damage of that sort that has been done so far. It was a bitter night, but we managed to block the hole up by means of a picture (in its frame) propped up by some of the wierd things that have been carried into the house.”
We don’t know from the description the exact window that got broken. Note, however, that there were relatively few hall windows, which narrows the possibilities. Also, the main stairwell window is definitely larger than the others windows visible in the one picture. But most importantly, Rev. Foyster included this incident in a report of questionable happenings.
The bottom line is that Borley Rectory evidently had two bricked up windows that were associated with possible poltergeist activity. I ran across another page in Fifteen Months In a Haunted House by Lionel Foyster. It says, "A stone through the window. . . .most certainly from the inside."
Given the Foysters were essentially standing below the window when it broke, the main stairwell window is indeed connected with suspected poltergeist activity as was the bricked up window in the dining room.