Quick and dirty waffle....The BFI at Sculthorpe was built in the 1950s as a result of the Cold War Era expansion of the GPSS (which had been largely decommissioned after 1946). Fed from from Thetford via Massingham AFDD. (This pipeline also had a spur before Massingham, to feed Marham). Prior to the new BFI and pipeline spur aviation fuel had been supplied to the airfield in bowsers and 'jerry' cans from Massingham. The water tank is as you suggest for fire-fighting. In military parlance an EWS (Emergency Water Supply). They were sited all over airfields, including the accomodation areas.
Another new pipeline extension from Thetford serviced Lakenheath and Mildenhall.