Limpley Stoke boatyard is the setting for Sid Towers's fatal investigation of the apparent locked-room mystery aboard Milo Motion's narrowboat. The boatyard brings together the Penny Black plot, Sid's curiosity about the padlock puzzle, and Diamond's reconstruction of the encounter that turned an elaborate scheme into a killing.

Setting and Association

Milo Motion kept his houseboat at Limpley Stoke boatyard. After a meeting of the Bloodhounds, Sid drove there; the police later told Polly that Sid Towers's car had been found at the boatyard. At first, his reason for going was unknown. The location mattered because Milo was away at the police station explaining the Penny Black, leaving the boat apparently available for someone to approach without interruption.

Shirley-Ann Miller reasoned that the person who went to the boat must have been one of the Bloodhounds: members of the group were the only people who knew Milo would be away. She said Polly had not expressed that conclusion aloud; Shirley-Ann had worked it out herself. The boatyard thus became an investigative link between the group's private knowledge and the otherwise unexplained visit to Milo's boat.

Sid's Visit and Diamond's Hypothesis

Diamond considered whether Sid had gone to Limpley Stoke out of curiosity rather than for a meeting with a blackmailer. Sid enjoyed locked-room puzzles, had trained in security, and had been told how Milo's padlock worked and why a spare key was supposedly impossible. The locked cabin presented a real-life puzzle of the kind Sid would want to examine for himself. At this stage Diamond posed the visit as a possibility, not an established fact.

Later, weighing the absence of evidence that Sid had received substantial blackmail payments against the elaborate circumstances of his death, Diamond developed that possibility into an alternative theory. The Penny Black's startling appearance had fascinated Sid, and he went to the boatyard to inspect the apparent locked-room puzzle. In Diamond's account, Sid was killed because he discovered what was happening there, not because he had been selected as a victim in advance.

The Boat, the Towpath, and the Killing

Diamond's eventual reconstruction places the decisive encounter beside the narrowboat: A.J. was replacing the padlock when Sid approached along the towpath. Diamond thought A.J. heard him coming, hid inside the cabin, and took the boat's windlass as a weapon. The plan had worked until Sid arrived; startled and frightened, A.J. struck him over the head, probably intending only to knock him unconscious. The blow killed Sid, and Diamond understood the actions that followed as efforts to cover up the killing.

The boatyard's importance is therefore not merely that Sid's car was found there. It is the point where the riddle's artificial locked-room challenge meets an unscripted visitor: Sid's interest in the puzzle brings him to the boat, and his arrival exposes the person changing the lock. The scheme's apparent cleverness gives way to violence at the very place where its mechanics were being put into effect.