Great game, loved it to bits. I want to do another playthrough, but the "delete save data" option doesn't seem to work. I'll press it and it does nothing. Is it broken or is there something I'm missing here?

can you just delete the save file after uninstalling? Do another install and see if that works? You will have to find where it saves the file too...usually somewhere in Documents, OR Local roaming.
Oh yeah, I guess you may have to do something about the cloud save if you have that enabled as well...pretty sure you or someone else can think of a workaround. Remember this game we have here is TWO PATCHES behind the steam version, this may have already been addressed there!

This was already fixed in the GOG version. They really do need to release that other DLC on GOG right now though...