Find the LR 5 Catalog Preview folder and delete it. The folder is called "Lightroom 5 Catalog Previews.lrdata". Lightroom must be closed and it is in the same folder as the catalog folder.
Thanks for the response, unfortunately I did what you suggested then reinstalled and it hasn't helped
Although I used a Mac and OS X for several years I gave up on it for just these type of things happening out of the blue. No rhyme or reason for them.
The only option I found to cure some of the glitches with OS X was to wipe the drive and start over with a fresh install of the OS and programs and or restoring the system from a known Good backup made with either Time Machine or one of the Disk Cloning apps available for OS X.
You could try using Disk Utility to do a repair of permissions that is found on the First Aid tab. That sometimes fixed issues like this.
It seems OS X sometimes forgets disk/file permissions and lock them as system, read only or restricted files stopping apps from opening or you saving files to a specific location.