The simple secret is to heat the area surrounding the screw in plug with the slot in it. I did this by carefully pouring boiled water over this area from an electric kettle. Obviously you need to ensure that you only heat up the screw in plug area and not get any water into any other area of the keyboard and damage the electronics. After heating the area simply unscrew with a coin. This worked first time and I removed the plug and then the corroded battery and then cleaned out the corroded area with no damage to the keyboard,