The floor of the hall was covered in sisal matting and lined with well-used-looking rubber boots, the walls hung with working gear-oilskin jackets and hats, bright yellow slickers, coils of rope. Smith led them through a door on the left into a sitting room as workaday as the hall.

The room was warm, if spartan, and Kincaid saw Gemma let go of her collar and take out her notebook. Smith stood by the window, still sipping from his mug, keeping an eye on the river. “Tell us how you found the body, Mr. Smith.”

“I came out just after sunup, same as always, have my first cuppa and make sure everything’s shipshape for the day. Traffic starts early, some days, though not so much now as in the summer. Sure enough, upstream there was a boat waiting for me to operate the lock.”

“Can’t they work it themselves?” asked Gemma.

He was already shaking his head. “Oh, the mechanism’s simple enough, but if you’re too impatient to let the lock fill and drain properly you can make a balls-up of it.”

“Then what happened?” prompted Kincaid.

“I can see you don’t know much about locks,” he said, looking at them with the sort of pity usually reserved for someone who hasn’t learned to tie their shoelaces.

Kincaid refrained from saying that he had grown up in western Cheshire and understood locks perfectly well.

“The lock is kept empty when it’s not in operation, so first I open the sluices in the head gate to fill the lock. Then when I open the head gate for the boat to enter, up pops a body.” Smith sipped from his cup, then added disgustedly, “Silly woman on the boat started squealing like a pig going to slaughter, you’ve never heard such a racket. I came in here and dialed nine-nine-nine, just to get some relief from the noise.” The corners of Smith’s eyes crinkled in what might have been a smile. “Rescue people fished him out and tried to resuscitate the poor blighter, though if you ask me, anybody with a particle of sense could see he’d been dead for hours.”