Hour One: the keypad
Nudge. The log is from the night in the blackout. What did you tell London on the telephone, at the foot of the stair?
Harder
In 1942 you gave London the stop-word that Mrs Reeve had hidden in five clues to Mrs Christie's books: the first letters of their answers.
The answer
The word is ember.
In Part 3 the drop to Linnet's field was cancelled at two minutes to midnight on the stop-word EMBER, decoded from HPEHU in the operations book.