On 3rd June I installed Norton for Mac as a trial and ran a full scan. Now every file on my computer has had it's Date Modified and Last Opened changed to Friday, 22 May 2015 14:45. I have no idea if this was due to Norton, but it is the only explanation I have as I have done nothing else that could have effected every file.
I have trawled the web for an answer but no one seems to have the same problem.