Have located it on 1:10,560 and 1:25,000 OS maps and on satellite photos. In water industry terminology, it appears to be a "spring source" - i.e. a spring that serves a limited number of properties not connected to mains water supplies. Depending on how many properties it supplies, it may be privately owned or it may be water authority owned; the fencing and general appearance suggests the former. If water authority owned, I would anticipate a chlorine dosing apparatus being present - these can be pretty rudimentary at small spring sources.
Whilst it may possibly be no longer supplying potable water to domestic properties, it may still be supplying water to farm animals. I strongly suggest that you make local enquiries before visiting it - the farms and houses down hill and along the contour would be the best to start with e.g. Babington, Dethick, Litlemoor, Low Leas. If it is still used, those supplied by it will not want you visiting it as you risk disturbing water supplies - it has been roofed to prevent detritus entering the supply.
Roofs of this type are usually very fragile and tend to be weakened if damp. Many people fall through such roofs each year, sometimes even when using crawl boards.