As Pace said - the coaling wharf for the CEGB Power station. When the place was operational, I remember it having a crane on it that had a grab which would be lowered into the boats on the river. Usually with mounds of coal on it too.
A modern picture here (I take it this is what you saw on your walk): Clicky
This is an aerial view of the power station in the late 60s/early 70s - the two chimney stacks of the power station itself can be seen smoking away, and the wharf is top right: