Hmm, maybe I spoke too soon: the internal SD card has filled up with CWM backups and other stuff, and nothing I do will get me any free space back. Deleting files using File Expert or other apps on the phone appears to work but the free space doesn't increase. Mounting the internal SD card to my Mac (using Doomlord's SD card mounter app) and deleting files that way achieves nothing as well. The files go, but the space doesn't come back.
Is this is bug with JB on the Acro S, or something specific to my phone - perhaps carried over from ICS because I upgraded using Sony Bridge for Mac rather than flashing an ftf to start from scratch?
Just reflash if you have unlocked bootloader... Try root explorer to delete files...