@declankeegan the very first thing you want to do is to replace the battery. Use something like [https://www.youtube.com/watch?v=_AsoYBUUhTM |this video for that. While you replace the battery check your circuit board for any corrosion damage, missing parts etc. Once you replace the battery, re-evaluate and let us know what you get.
Take lots of pictures. Since iFixit does not have a guide for this, use those pictures and create a guide [guide|21499] The community will appreciate it!