IMHO very unlikely to be a direct part of the fuel installation, if you look at the overfly picture you can see the fuel installation to the left of the red ring. IIRC This was a standard installation with the tanks buried in a concrete tank room in the centre of the grassed area, the pump room, again buried or partially buried was to the front of the tanks and there were was a single receipt and dispense gantry on each side. Also, if it was then where is the building near the other, southern BFI?
DIO or rather DE as it was back then should still have the project file from the works that were carried out circa 2001/2 to remove the old BFI's. (Copies were handed to the DE Property Manager & RAFIO by the EWC when the project was completed and they signed for their copies!)
The BFI's were all photographed and recorded before demolition as a means to historically record the design, location etc in context with a expansion period airfield. The access roads and grassed areas for the BFI's were left in place to aid context etc. Again the DE Property Manager & RAFIO all had copies along with the Cherwell District Council Conservation Officer (and yes, they signed for them! I didn't make people sign for the biscuits in the meetings, but pretty much every thing else was fair game...)
At a guess... I would consider:
A. Some form of pill box, IIRC there's a crash gate just off the right of it. There is a complex of pill boxes, trench and command centre used for the ground defence of the airfield running along the southern boundary of the site (2002). May have been something similar.
B. Water borehole or pump room, I know of one in a pump room on the Tech Site that was pretty close to the hedgerow and another one up the A4421 on the other side of the road in a brick pump house near the turning into the old O/Mess. The location of this Bldg would put it in line with those two assets.
C. Electrical Sub-Station, outside chance on this one mind, in 2002 the main intake (HV) was on the domestic site, with a radial to the old SBSH located in the Tech Area with LV Feeder Pillars run from there covering the Tech Area. IIRC there was a LV Feeder Pillar in the grassed area about halfway between the Northern End of the smaller hangar and the BFI.
D. Has any one considered something going back even earlier? There was a presence on site from 1918, if you look in the grassed area about 20 to 30 mtrs in from the boundary of the A4421 at the northern edge of the airfield you will find the remains of a Bldg. IIRC early maps of the site indicate the 1918 airfield having some hangers in that area.