We quickly saw that the reason that this mount was full was that RMAN had tried to make another set of image copies in the latest backup run rather than take an incremental backup for recovery. It does this when it finds no valid copy of the datafiles to increment, and the logs confirmed this to be the reason:
Starting backup at 2014/02/26 13:30:16
no parent backup or copy of datafile 792 found
no parent backup or copy of datafile 513 found
no parent backup or copy of datafile 490 found
no parent backup or copy of datafile 399 found
... and so on, for every datafile. However I knew that the copies that had been there (and been updated) every day were still there. So what was different?