The lift stopped at the third floor. Holmes pulled the large iron gate aside and beckoned for us to step out of the lift. We were faced with a solid looking oak door and along a long wall to the left of this was arranged a curious mixture of pots and stands of varying heights. A profusion of plant life filled the space and of those I recognised I could make out Kentia Palms and Sword Ferns and at least one large Yucca.

“A pretty display, eh, Inspector?”

Lestrade smiled weakly. “Yes, Mr Holmes. An odd assortment if you ask me. And exactly the same display on each of the first four upper floors.”

“Patently. And on my earlier tour with the concierge, I was told that Mr Cartwright had paid a handsome sum for a local horticulturist to come in and assemble the collections only a week ago. Just before Miss Trelawney started work in fact.”

“And is there any significance in that, Holmes?” I asked, unsure where he was heading with his observations.

“Yes. This was a key part of Mr Cartwright’s diabolical scheme. Let us head up to the fourth floor and you will see why this was important.”

Some minutes later we re-emerged from the lift to face an almost identical scene to that we had enjoyed on the third floor. Holmes headed towards the door of the office and turned to face us. “You will note how similar the two floors are. I have no doubt that when Miss Trelawney approached this entrance yesterday, accompanied by her very chatty employer, she was convinced that she was on the third floor.”

We followed him into the outer room of the office. It was sparsely but expensively furnished and my eye was drawn immediately to a large panelled door on the far wall. Holmes followed my gaze.

“Yes, Watson - that is the door through to the room which Mr Cartwright occupied for most of yesterday morning. Had it been his regular room on the third floor, he would have had no means of reaching the stairwell or lift without being observed by his secretary. The only exit from that office is through the door in the outer room. But here on the fourth floor, he could take advantage of the one architectural feature that distinguishes this office from those on the lower floors.”

At last, I understood what Holmes was alluding to. The four of us filed in to the back room where we could see to our left another panelled door - one that provided an alternative exit to the lift and stairwell.

“Very ingenious, Mr Holmes!” cried Lestrade, pointing to the door. “So that is how chummy here managed to reach the stairwell without being seen. It was then a simple matter of making his way down the stairs for the encounter with Edward Flanagan.”

“That is correct. Violet Trelawney was unaware of the door because she was convinced that they were on the third floor. The careful positioning of the plants and stands outside the office has disguised the second exit - a task that was commissioned just before she began her new role.”

“And yet, she knew instinctively that something was amiss,” said I, unable to curb my excitement. “Holmes, you may remember that she confessed to feeling somewhat giddy while she sat in the room and imagined that the walls were closing in on her. It was less than twenty years ago that a medical colleague of mine, Dr Benjamin Ball, first coined the term claustrophobia which is now used to describe this feeling of anxiety. But in Miss Trelawney’s case there was a particular reason for her discomfort. With the additional space taken up by the hidden corridor, the room in which she found herself was genuinely smaller than that on the third floor.”

“Quite so!” agreed my colleague.

Inspector Lestrade moved a little closer towards Cartwright, who was looking considerably less composed than he had earlier. “But what about the gun and the coat and hat?” he queried.

“Where he left them yesterday, Inspector. I apologise for having badgered you earlier, about which areas of the building you had searched, but was fairly certain that our killer could not have disposed of the weapon or his disguise before your arrival yesterday. I took the liberty on my previous tour, but if you care to take a look in the large desk drawer to your left, you will see all of the offending items, including the revolver, tucked away at the bottom.”

Lestrade stepped across to the desk. His face lit up as he opened the drawer and saw the evidence. “Well I never!” he uttered, retaining a keen eye on the increasingly fretful Cartwright. “I did think it was odd that our friend here should have made so much fuss about hearing the shot and insisting that he and Miss Trelawney trek down to the ground floor to speak to the concierge - especially when the young lady herself was minded to ignore it.”

“Yes,” agreed Holmes, “all part of his plan to create the illusion that he had been working in the back room all morning. Having shot Flanagan, I imagine it took him a few minutes to run up the three flights of stairs to the fourth floor, remove his disguise and hide both it and the revolver in the drawer. What was most telling was that he should be so keen to pick up Miss Trelawney’s work tray in leaving the office. He believed he was removing the last trace of their presence in the fourth floor office. But, of course, he was wrong.”

“I’m not sure I follow,” said Lestrade.

I could not resist stealing Holmes’ thunder. “Why, the curious matter of the missing Pearmain, of course.” I looked across at Cartwright. “You didn’t know anything about the apple she had hidden in the desk drawer, did you?”

Cartwright scowled and then raised his head in defiance. “No, damn you! I did not.”

“And I think you’ll find it’s still in the drawer,” Holmes mused. “A small detail, but a significant piece of evidence which will help to seal Mr Cartwright’s fate. As for his motives, Lestrade, you might like to look into his business dealings in recent months. I understand he is close to bankruptcy having lost a small fortune investing in a property venture in Canada which has collapsed as a result of the recent Newfoundland Bank Crash. The last thing he wanted was a tenant who refused to pay the extortionate level of rent he demanded and one who seemed determined to upset the other occupants of the white elephant that is Ravensmere Towers.”

Holmes’ barbed comment prompted an angry snort from Cartwright, who stepped forward clenching his fists. Lestrade barred his way and stood eyeball to eyeball with the property owner until Cartwright stepped back, realising he had been outwitted. The inspector then produced a sturdy pair of handcuffs and secured his wrists.

On our way out of the office Holmes stopped to open the drawer of what had been Violet Trelawney’s desk the previous day. As he had predicted, the apple lay where she had placed it.

Inspector Lestrade was ecstatic when we reached the lobby of the ground floor. He pushed Cartwright towards PC Clarke, who took charge of the dejected prisoner. James Mount looked on with evident discomfort, unsure whether he should be assisting the police officer and clearly troubled to see his employer in handcuffs.

Lestrade turned to the two of us as we trailed behind. “Mr Holmes, Dr Watson. I cannot thank you enough. I will, of course, mention your invaluable assistance when I speak again to the press.”

Holmes responded in a hushed tone. “My dear Lestrade. I would much prefer it if you kept my name out of the papers on this occasion. The intelligence about Mr Cartwright’s business affairs is not common knowledge and I would not like to have to reveal my source. I think it would be better for all concerned if you were to take full credit for the investigation.”

The inspector could not have been happier. He accompanied us to the entrance and opened one of the large front doors before bidding us farewell. We stepped outside into the biting chill and thick acrid smog of the London air. Holmes took the lead, striding off towards Hyde Park, his eyes and ears alert to any sound of a nearby carriage. I pulled my coat collar up around my neck and shivered as I walked briskly to keep up with him.