LTNMAN,
Trying to think back in the past when I worked next door to that hangar and now you mention the steelworks. I have a feeling that there was going to be a second hangar ( never built ) but you might well be correct in that the steelwork perhaps was erected.
I am certain that the hangar was originally built for the newly formed company London Aviation Centre but now thinking back maybe the offices either side was added on a little later when L.A.C. ceased and there was a need for more offices on the airport.
As said earlier, there was a company importing Mooneys based in the hangar and I think perhaps Delta Helicopters also relocated into the hangar/Offices.