It is strange that you mention a dilapidated office block in Brentwood.
I argues with a friend in work that Amstrad is based in Brentwood, but even after showing them Amstrad's website and where they are based, they still argued that SAS is based at Canary Wharf.
I said that Canary Wharf is just used for The Apprentice as Amstrad's Brentwood offices are hardly a picture of inspiration.