Geograph - photograph every grid square

This small windowless building stands on a grass verge between Billy Mill Lane (in the background) and Barnstaple Road (in the foreground). It is fitted with a television-type aerial, and has a telegraph pole close to it, with a number of wires going off in various directions. I thought it may be a telephone exchange, but I am advised that this is unlikely. It would seem more probable that it has some purpose connected with the nearby underground reservoirs. Further suggestions gratefully received!

Update, Jan. 2021: I have received a message from a gentleman who lived locally to this little building. He tells me that he was taken into it on a school visit in the 1960s as a pupil at a nearby school. He says, "The building certainly has to do with the nearby reservoirs - or did have. It was full of meters and dials and those moving pen things that scratch lines on moving graph paper! All to do with the flowing of water at the nearby reservoirs - which I believe are now redundant as housing is being built over them!"

Apparently younger children called it "the fairies' house".