Docklands railway is currently up the spout (as I found out the hard way today...) due to massive refurbishments and new engineering between Tower Gateway (by the Tower of London) and Poplar. Essentially you can only travel between King George and Lewisham. This is long term works.

But there is an Ibis at Aldgate, so you wouldn't have to be all the way out at ExCel anyway *G*.