Milford Haven.

[Engineer]

After studying the pics of Buxton, where the pumphouse entry blockhouse appears to show stairs down in two directions at 90 degrees, I remembered this write up.
May be of interest.

http://www.subbrit.org.uk/sb-sites/sites/m/milford_haven_aviation_fuel_tanks/index4.shtml

[PNK]

There is an excellent aerial of the Milford Haven sites in the relevant volume in Ivor Jones' trilogy on "Airfields and Landing Grounds of Wales". There is also a closer one of one set of tanks.

[Engineer]

I'll see if I can get that book added to my Xmas list.