Geograph - photograph every grid square

While I understand the basic processes of our water supply system, the nuts and bolts of it always remain something of a mystery! This was the best explanation I found online :
'A washout hydrant is owned by a water company and is an accessible valve in a water supply main that is used to run dirty water out of a system following repair work. Its metal cover is usually marked with a WO.'
For a fee it seems, reading the various water company websites, they will let you connect a standpipe to this supply for certain approved tasks - landscaping, building work etc.
As well as the two drain covers in the foreground there were five more in the fenced off bit, as well as seven of the little warning mini-tombstone things. This being the source of the dirty water, presumably it is a serious business at this end. Hidden from view is one of those dark green mysterious utiliy boxes!
Most covered reservoirs look identical and I noticed on online maps there was a four metre height difference between the reservoir top and surrounding land so guessing that's the UK, or Scottish standard.