"RAF Bicester" by Paul Francis, Airfield Research Publishing (pub. 1996), seems to indicate that this is Building G (ie not allocated an AM building number). It is described as a small brick and concrete air raid shelter close to the North Aviation Fuel Installation (Building No. 72), for use by personnel caught using the latter at onset of an air raid.
There was indeed a Battle HQ, part of an a/f defensive system in the North Sector "between the North dispersal track and the North Sector of the perimeter track, for co-ordinating a/f defence during invasion. It was surrounded by five pillboxes. That doesn't quite tally with the OP building I would suggest.
