Lived in Hempsted as a kid in the 80s and early 90s and walked past this most days. Looked well maintained then, and the other depot was derelict but still existed. I remember looking around the derelict one but never knew what it was back then and didn't own a camera. From the bottom of the recreation ground you could get down into an old railway cutting (now a road) and follow it to where it used to cross the canal, then get back up to where the tanks were. It's visible on old google earth images not far south of the depot in this report, and pretty much in line with the pipeline shown at the bottom of the first picture.
I don't remember seeing any sign of a pumping station.