What is it? - the answer

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.

The breather (if that is what it is called) is marked "Huddersfield Canal Society".

The photos are from 28th August 2015.