Mrs. Hudson's stern-door high-security padlock

Mrs. Hudson's stern-door high-security padlock turns the murder of Sid Towers aboard Milo Motion's narrowboat into a test of the difference between a sound lock and a reliable account of how it was used. Forensic examination found no defect or sign of tampering, and the lock's apparent security helped make the cabin seem impossible to enter. Yet evidence about Milo Motion's lost keys eventually exposed the assumption behind John Wigfull's solution: the padlock had not been substituted as Wigfull supposed.

Design and forensic examination

The stern door was fitted with a fresh lock, which a constable had to open to admit the investigators. The padlock associated with the boat was German-made and described as a high-security close-shackle model. At Peter Diamond's prompting, the forensic laboratory dismantled and examined it. The scientists found no flaw in its mechanism and no sign that anyone had tampered with it. Its construction therefore supplied no physical explanation for how an intruder might have bypassed it.

The lock's security was expressed in unusually concrete terms: its mechanism offered more than six million key variations. That figure supported the manufacturer's claim that every padlock sold in Britain had a unique key pattern. In the investigation, this was not simply a claim about sturdy hardware. If the boat had been locked with that padlock and Milo Motion's key was the only matching one, ordinary access by another key became difficult to reconcile with the evidence.

The locked-cabin problem

Peter Diamond repeatedly reviewed the narrowboat's security arrangements without finding a way to explain how Sid Towers's body had come to be inside. The padlock was still in position when Milo Motion returned with John Wigfull; Milo opened it with his key, and the two men discovered the corpse in the cabin. The lock thus seemed to preserve the very condition that made the death puzzling: the door was secured from outside, while the other entrance, at the bow, was bolted from inside.

That reconstruction depended on Milo Motion having told the truth. On that account, he had locked the boat when he left, kept the key with him, and had a key that fitted this padlock and no other; there was no second key. The fore-end door's internal bolt closed off the only other stated route into the cabin. Even with the hardware showing no defect, Peter Diamond could not treat these reported circumstances as proof that no one had gained access. He returned to the qualification that Milo's account had to be tested. The lock narrowed the mechanical possibilities; it could not certify the human story surrounding it.

Wigfull's proposed solution

John Wigfull made the padlock the showpiece of his explanation to senior officers. He presented it as evidence of the boat's security while describing the theft of the Penny Black cover as both a theft and a planned stunt. In his account, someone with inside information knew Milo Motion would bring his copy of The Hollow Man to a Bloodhounds meeting about locked-room mysteries. The missing cover appeared at the chapter devoted to that subject, and the book had supposedly been aboard the locked boat all week.

Wigfull's proposed solution turned on a replacement lock and a spare key. He argued that Sid Towers had fitted a new padlock and would therefore be able to visit the boat whenever he wished. That theory supplied a way around the apparent barrier: if the lock had changed, the key Milo carried would no longer establish who could enter. Peter Diamond initially acknowledged the explanation as persuasive, while noting that the murderer must have taken the key from Sid Towers's body. The exchange showed how completely the padlock's identity mattered: the locked-room account could stand only if the lock-switching premise did.

The recovered keys and the reversal

Later, after divers had finished their work, Peter Diamond took a rusty bunch of keys from his desk and identified them to Julie Hargreaves as the keys Milo Motion had dropped into the canal. When she asked whether they were Milo's original keys, he confirmed that they were. Their recovery changed the interpretation of the padlock. If the keys were original, the boat's lock had not been substituted in the manner John Wigfull proposed; Peter Diamond concluded that it must be the original padlock and that the investigators had been working from a false assumption.

The discovery did not itself explain how Sid Towers was killed or how his body entered the cabin. Its consequence was more specific, but decisive: it removed the replacement-lock mechanism that had made Wigfull's account work. The padlock remained physically sound, but its evidentiary meaning changed. First it seemed to certify the cabin's security; then it became the hinge of a plausible but mistaken solution; finally, when set beside Milo Motion's recovered original keys, it exposed how a sound object could support a faulty reconstruction. The mystery shifted back from the lock's mechanism to the truth of the events and testimony surrounding it.