Argh argh argh. Not sure if my problem's related, but I assume so. My download of 4.2.1 had an error and was unable to complete, wiping the phone. Fortunately I had backed it up and when I reinstalled, it worked fine. Of course, my music had disappeared completely so I panicked. Nothing in iTunes and was saying I had almost all my memory free. Tried the fix above, but didn't work so I went through all my iTunes and put all the music I wanted on there (very laborious but oddly cathartic).
However, I've noticed that not only did it clear all of my music, but all my comics through the Comixology, Marvel, DC and Image apps, so I've had to redownload that as well.
But the strange thing is that before I had about 14GB on the phone, whereas now with minimal differences I've got 29GB on there and am almost out of space. Conspiracy theory time from me but does anyone think that I might have put my music on again unnecessarily and have it on there twice (albeit in a hidden form) thus accounting for the leap in space used?