Looks like it may be some sort of plant room or pumping station. The lifting beam proberbly for lifting in/out motors or pumps. Just a guess from similar type buildings i've worked in
Imagine a dockside crane that moves about on a pair of rails parallel to a dock side (as here). The crane needs an electric power source, and therefore a building like this is put up on the dockside, to contain a big reel of power cable that unwinds, and rewinds as the crane moves up and down the dockside.
It therefore has an open doorway between the crane rails on the dockside (like this building - ie the lower door), and these buildings also have (as usual) an upper doorway, with a girder sticking out to enable them to winch plants, electric motors etc , in and out for maintenance.
These days, the crane power lead is often attched to the crane, rather than the dockside, so that teh cable does not scrape up and down the dockside.
The previous post of mine may have been a red herring (but a good one). Looking at the Cheshire CC 1970 aerial photos:
The warehouse to the east (ie towards the lift bridge) appears to have overhung the dock quite a way, so it was only high enough for barges underneath the overhanging roof.
The warehouse to the west appears to have a covered conveyor belt coming from it's roof, to underneath the overhanging roof of the eastern warehouse.
This building may have been the bottom end of the conveyor.
Inbetween the front and back wharehouses was a dock, the entrance to the dock was between the west and east whatehouses. There was a dock gate (still visible) and this building was right beside the dock gate. Looks like it may have been connected - may have controlled the dock gates.
The conveyor between the buildings was high up, if I remember correctly - there are some pictures somewhere.
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As far as I can determine (and remember) the high level bridges, of which there were many, were walkways not conveyors.
However, there was a hydraulic winch system installed along the dock wall which was used to pull barges between mill and warehouse. The building could be related to this system (a pump house?) it appears to have windows on the dock side of the building which would allow them to see the barges being moved?