Chandler-Powell said, “It would be perfectly possible after she had fully regained consciousness, but as she was constantly monitored while she was in the recovery room, and initially visited every half-hour after she was wheeled back to her suite at four-thirty, the only possibility would have been after ten o’clock, when she had been settled for the night. Then, in my view, she would have been physically capable of leaving her suite, although there would, of course, have been a possibility that someone would have seen her. And she would have needed a set of keys. She couldn’t have taken a set from the key cupboard in the office without setting off the alarm. This map of the Manor shows how the system works. The front door, the great hall, library, dining room and office are all protected, but not the west wing, where we rely on bolts and keys. I am responsible at night for setting the alarm, Miss Cressett when I’m not here. I bolt the west door at eleven unless I know someone is out. Last night I bolted it at eleven as usual.”

“Was Miss Gradwyn given a set of keys to the west door when she arrived for her preliminary visit?”

“Certainly. All patients are. Miss Gradwyn inadvertently took her keys with her when she left. It does happen. She returned them with apologies within two days.”

“And on this visit?”

“She arrived on Thursday after dark and said she had no wish to go into the garden. In the normal course of events, she would have been given the keys this morning.”

“And you keep a check on them?”

“A reasonable check. There are six suites for patients, and six numbered keys with two spares. I can’t vouch for every set. Patients, particularly long-stay patients, have freedom to come and go. I’m not running a psychiatric hospital. The west door is the one they use. And, of course, all members of the household have keys to the front and west doors. These are all accounted for, as are the patients’ keys. They’re in the key cupboard.”

The keys were in a small mahogany cupboard fitted to the wall beside the fireplace. Dalgliesh checked that all six numbered sets were there with two spares.

Chandler-Powell didn’t question what possible reason Rhoda Gradwyn could have had for arranging an assignation when postoperative, or the many objections to any theory based on this unlikely hypothesis, nor did Dalgliesh pursue the matter. But it had been important to ask the question.