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 dock workers have fenced this area off for ages with no sign of them actually fixing the foot bridge.
So the cache area is fenced off, so in the short term, or possible long term 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