Peter Hutchinson, since you are possibly dealing with a water damaged phone, I'd start by cleaning the phone's logic board. Use [guide|5825|this guide] to clean it. It was written for a 3G but still pertains to your phone. After that, re-evaluate your phone. It is possible that it is your LCD that needs to be replaced. If that does not resolve the issue, it may be time to check the backlight coil 6R8 or the backlight IC 109B. Check [http://www.ifixit.com/Answers/View/50750/Where+do+I+buy+the+backlight+coil|here] for a location of the coil. Hope this helps, good luck.