Apparently there is a registry fix that is supposed to address this issue of Old backup sets not being deleted.
http://www.symantec.com/business/support/index?page=content&id=TECH187957
Just did this to my server today as I again am looking at 10 mon...
Easiest way I have found (Still not pretty).
Select the Storage Tab in the BE2012 Interface.
Select your disk storage device and click the ====> for the DETAILS view.
in the details view select the Backup Sets option.
I just scrolled to the bottom ...