This "apron" was parallel to the Slough (name?) Rd.
?Bath Road?
I was based at LHR '89 onwards and am trying to visualise where the picture provided by the OP fits in with my much ore recent model of the airport..
With regard to the image in the OP - Am correct in thinking the minor road with a bend in it heading away from the viewer, centre view towards the top, is the Sipson Road, and if that's the case the Terminal being talked upthread was situated somewhere near the current Renaissance Hotel (I think that's what it's called this week)?