The first location will give you the clues to the final cache location. To fill in the co-ords of the image below.
(if your on the mobile app click on Web View to see the image)
The co ordinates will take you over tower bridge to see some other bridge history.
---------UPDATE------------
The old Mayor building is getting modernised and the first cashes may be blocked off.
Luckily there is a twin if needed N 51° 30.208' W 0° 04.764'
The dock workers may have fenced this area for repairs.
So tif he cache area is fenced off, I have put a duplicate cache on its twin.
So go to the Cache location as normal then look for its twin, which is close by, and also not ;-)
When you do find its twin you can read about what your looking at on the plaque.
Cache container is the same on both sites, and is just as hard to find.
I can send you an location image if you get stuck hunting for the one in a million.
Happy Caching