Mark Two: The Knocks on the East Wall
Nudge. Mrs Marsh says the spirits are spelling the boy's name. Find the run that would spell KIT, and it gives you the alphabet. Then: Cyril could only knock while nobody could see him.
Harder
11 9 20 is K I T if one knock is A. With Cyril in plain sight at 9.12 and 9.13 the knocks were 16 8 15 and 5 2 5, the same runs as at 9.04 and 9.05: sixteen is P, eight is H, fifteen is O.
The answer
The word is phoebe.
Cyril stood in the doorway at 9.12 and 9.13, so the runs then were not his: 16 8 15 and 5 2 5, repeating the runs at 9.04 and 9.05. On the spirit alphabet they spell P H O E B E. The 11 9 20 runs, K I T, came only while Cyril was out of sight: that is the nephew in the darkroom, knocking the name his aunt wants.