Well looks like I solved the problem. Apparently most people are seeing this message, and you basically do have to add something on to the end of the name. So what I did was rename myself in Google+ and shorten my name by a few characters, then I added *those* characters to the end of my custom URL, allowing me to get the exact URL I wanted. Then after that I changed my name back in Google+. Now the custom URL is working and I have my existing name still
Tl;dr if anyone faces this problem, do the following:
Say your channel name is "BobsHatStore" and you want your custom URL to be "BobsHatStore", but when you try to do it, it tells you that others have this name, and to add a few characters/letters. Yet you've checked and confirmed no-one else is using this name on either G+ or YouTube.
Now..
Rename yourself in google+ to "BobsHatStor"
Refresh the page, click "claim custom URL", it'll say YouTube.com/c/BobsHatStor and then it'll tell you that other people have this name and to add a few characters or letters.
So just add an "e" so it should now say:
YouTube.com/c/BobsHatStor - e
giving you the url of YouTube.com/c/BobsHatStore/
Now accept the terms, confirm. Then go back to the Google+ name change option, and change your name back to BobsHatStore. Voila. Job done.