I have seen double rainbows in the past but I have never seen the rainbow being reflected in this way by glass windows.
It would appear that the end of the rainbow is in the East Wing of Newham Dockside. I think in the Facilities Management section.
Newham could definetely use a (large) pot of gold. I wonder if Terry & Conrad will be there with their pick and shovels first thing tomorrow? I would expect nothing less.