So I installed a fresh Kubuntu 15.04 on my desktop and all was going smoothly (ish).
I noticed that whenever I tried to move a program in my favourites list or add a new one, the whole Kickoff menu, the panels, everything just froze.
I found a fix.
There was a lock file,
~/.config/kickoffrc.lock that hadn’t been removed (I guess during some other edit I made) – I simply deleted this file and everything started working again.
Hope it helps someone else who has this problem!