I love the Docklands! It's an area rife with history, and these permanently fixed cranes are a reminder of this. The Docklands wasn't built until mid 1800s, to deal with the congestion on London's river. The intention was to build bigger and deeper docks, able to deal with the now larger vessels and demand of goods.
And, bigger ships meant a need for larger machinery to deal with the weight and volume of their loads. Hence these massive, permanent cranes were built. They could easily move goods from ship to warehouse. Sadly, they weren't in use for very long because soon the ships became too large even for these kinds of docks... aaah capitalism :)
The Docklands are dotted with nice natural areas. Close to this cache is Mudchute and Island Gardens. You'll also find loads of nice places to eat and drink. Please note there are usually people around so some stealth skills are required. If you're going by public transport Crossharbour or South Quay are your closest DLR stations.
Anyway, you're looking for a micro cache, that blends in pretty well in its surroundings. You'll probably need tweezers to get the log book out.
I'm not sure this is entirely wheelchair accessible. The area in itself is, but I'm not sure picking up the cache is.