Kevin T of NB Tranquility got it. It is the breather for the ground paddles on a lock on the Huddersfield Narrow Canal. The pepperpot-shaped device seems to serve as a bollard or strapping post too.
This is at Lock 12W.
Middlewich Folk and Boat Festival - *Sat 16th Jun* Aqueduct Marina We drove to Middlewich via Church Minshull, with instruments in the boot, and we parked in the car park for the Boars Head...
5 hours ago