One of the contributors to my radio programmes about East London said that Margaret Thatcher had been very annoyed when the Independent revealed that the pyramidal roof of Canary Wharf concealed radio transmitters belonging to MI5.   I can find no evidence of this.   But what I did discover, which I had never recognised, but does sound plausible, is that its profile and distinctive shape is modelled on that of Big Ben;  and that the pyramid conceals the machinery for washing the windows:-