What do you mean by "It would appear that the symlink in /home/public/data/pvr that would normally point to my second hard drive has "turned itself" into a folder"
Normally there would be a symlink in /home/public/data/pvr called "STB 7823493-whatever" that points to the pvr folder on the second HDD. However, what I found was that the symlink had disappeared, and been replaced with an actual folder called "STB 7823493-whatever" (on primary HDD). This folder contained a symlink called "pvr" which pointed to pvr on the secondary HDD, which didn't look right at all.
And it was fairly obvious to me that the recordings were still going to "/home/public/data/pvr/"STB 7823493-whatever" (now a folder) because it was filling up my little hard disk.
Despite being quite worried that the whole thing was going to go willy tits, putting the symlinks back the way they should be solved my problem. I still have no idea what could have caused this in the first place.