*_Possible solution_*
Like a idiot I left my screen on for a little over 18 hours, when I returned to my iMac all the disk icons, folders, dock, menu bar had all burnt into the screen.
I searched and searched for a possible solution and after a few hours I managed to find a solution which did correct my screen.
A few websites recommend watching a few full length movies using the iMac, well I did this and after 2 hours worth movie there was a slight visible difference. The next was to use a screen saver called LCD Scrub. I used LCD Scrub with the 'High Contrast' selection running for 8 overs (over night) when I awoke this morning, I immediately checked my screen and everything was back to normal, screen burn gone.. everything back to normal.
So, to summarise
1. Watch or just leave a DVD/Movie running
2. Use LCD Scrub - following instruction in the screensaver field
(LCD Scrub =
http://toastycode.com/lcdscrub/ )
As mentioned, the above method worked for me