Oh, London City is lovely!
Get the Circle / Hammermith & City / Bakerloo (Circle is probably better as I think more frequent, slightly faster and less walking) to Baker Street, and jump on the Jubilee Line to Canning Town, then the DLR.
I'd leave a good hour and a half, to allow for waiting for the DLR, which aren't as frequent as the tubes. Allowing 4 mins per stop, plus 4 mins per change, comes to 76 mins, so you may actually need less.
Vicky (tube geek and better than TFL Journey Planner
!!!)
*edited cos I forgot the Circle line! Doh!