Missing space/Storage space running out Galaxy S2 [tried almost every solution I can find!]
Just so anyone reading this who's having trouble knows, I figured out the answer.

My phone has several data/log folders (for some reason), and definitely had saved dumpstate files to those locations before so I assumed that when they were empty that wasn't the issue. HOWEVER. 

I thought I had looked everywhere on my phone, but using ES File Explorer, even with 'see hidden files' turned on, doesn't show you all the files on your device automatically. Turning on Superuser lets you view more files, and there was another data/log file full of 1GB of dumpstate files hidden in another folder. Deleting this sent my storage space back to normal.

So, if you've tried deleting log files with a file explorer app and it hasn't worked, check your settings, make sure your file explorer app has all the access permissions it needs, and look harder. They're probably there.

Knew I was doing something wrong! I had never read anything in existing answers saying that this might happen when you look through your phone with a file explorer app, so hopefully this helps someone who's still stuck (that or it was just me).