I suggest you question the local planning authority (LPA) in the new year.
This is clearly 'development' and, therefore, requires consent. It is possibly what is known as 'permitted development', i.e., the structure does not require specific planning permission, which would probably be the case if, as suggested, it was necessarily erected by the water company or some other statutory body.
Nevertheless, should the 'lawfullness' of a development be questioned, the LPA has the wherewithall (and duty) to investigate and respond - which they will - and/or take enforcement action should there be a breach of planning control - which they will not unless the rogue structure happens to upset a councillor on the planning committee or some other worthy, but that's another story.
Seasons greetings