Dell Vostro 1520 :: Battery Not Charging?

I have a Dell Vostro 1520 supplied to me direct from Dell.  I have always used the laptop on the mains adaptor until recently when I needed to run it off the battery and found it not to work.  On the front of the laptop the battery charge light flashes from orange to blue continuously.  The laptop still runs ok from the mains adaptor so I am guessing either the battery or the adaptor are faulty in some way, although the adaptor looks fine and fits snugly into the mains port on the laptop I have heard mainly people complaining on forums that they have had problems with their adaptors with no sign of damage to it.  Obviously I don't want to replace the adaptor if it's a battery problem and vise versa.  I have done the laptop self test on the battery which just comes up with a general battery fault because it has no charge. What the orange to blue flashing light indicates.