Had this phone since 2008 and only use in emergencies etc. However, am about to be without a landline for a few months so will use the mobile more. Today when I charged up the screen has gone dim. It was perfectly ok yesterday. I still have about £40 credit so don't really want to buy another (I intend to change from O2 to 3 mobile if I have to replace it). Is the problem due to the battery failing? If so can I buy another battery and would it be expensive?