Very interesting! Looking on flashearth for Heston I found this -
http://www.flashearth.com/?lat=51.48....1&r=0&src=msa It looks like a hangar and it sits at an odd angle to all the other buildings. It also seems to match the one in this photo
taken in 1934.
If you swap from Microsoft VE to Googlearth on your Hanworth link then I fear that we have seen the last of this circle.