I've often wondered where it was and if it still existed. Turns out all I had to do to find out was send an email to the British Water Tower Appreciation Society.
Within hours I had this reply, from Ferrers Young, archivist for the BWTAS:
"Thank you for contacting the British Water Tower Appreciation Society — no need to ask the membership, as I recognise that one as Tadley water tower in Berkshire — 750,000 gallons of loveliness…
Constructed in 1975 at a cost of £271,000 for the Thames Valley Water Board. Design was by Scherrer & Hicks and consulting engineers Kenneth Severn & Partners produced the structural design, construction was by Leonard Fairclough Ltd. The tank top is flat with no penthouse and it’s base slightly conical, is supported by an octagonal central service shaft with an internal diameter of 15½ ft. and eight inset tapered legs, 46 ft. high, being wider at the top, being an irregular hexagon in section. The tank is 122 ft. 7 in in diameter and contains two concentric compartments. The normal working water depth is 12 ft. with a maximum depth of 17 ft. 2 in. The tower is only 60 feet tall and has a top water level of 405 ft. above Ordnance Datum.
The tower is extant, as far as I know (still seen on Google Earth™) and located at O.S. Grid Ref. TQ 23319 55697 — 51.361154,-1.165839 for Google Earth."
Sure enough, here it is on Google maps:
Roughly 8 miles north of Basingstoke, between Newbury and Reading (and down a bit).
Many thanks, Ferrers!
I heartily recommend the British Water Tower Appreciation Society blog, which has interesting pics and stories going back to 2008.