I will tell you, it was the Shard.. London's very own glass pyramid caught the suns rays at an exact alignment allowing it to blind both my eyes when I looked out the window.
I quickly grabbed my camera, took off the lens cap, switched on and [NO CARD IN CAMERA] arrrg! I knew that this may not last so I threw in a spare card and took a few shots.
Take a look at the other shots here, I took one at 1/4000 of a second where you can see individual windows.
It only lasted around a minute so I am glad I had the camera ready.