Problem Solved - what a waste of time!
I tried all the suggestions on this thread but nothing worked - downloading Desktop Lighter seemed the best option but it still wasn't great as only changed the colouring to give the effect of lighter and darker.
Now I still can't use the function keys for brightness but this solution is so simple. I ended up asking my guys at theofficeproviders and they showed me instantly, felt pretty stupid as tech isn't even what they do but hey ho! they didn't make me feel too bad!
Ok go to 'Control Panel' , hit 'Appearance and Personalization' , hit 'Display' , hit 'Adjust Brightness' . Now I kept going in to the Power Plan options and messing around in there for ages. Ignore those options look down at the bottom of the main window and you will see the Screen Brightness scroller. I never saw it because I was being distracted by the powerplan options.
How simple is that?! Mine's a Sony Vaio, hope it's as easy on every Vaio. Mine's a VPCCB2MOE bought December 2011.
Hope it helps...