I managed to get 2011.20 and it was my fault that I missed it, I hadn't scrolled down far enough.
If you log on to myGarmin, plug your Zumo in and go to myDashboard, it should offer you 2011.20, and it can be downloaded more than once if it doesn't work properly first time.
You have to select where you want it loading to though, I selected Zumo and computer so that it would update my MapSource maps as well.