@niall.lawrence… it sounds like the battery may have lost its capacity to store energy. i say this as i am taking into consideration that it shows that it is charging via the indicator light. with that said, it is very possible that even though it shows the indicator that it is charging, it is not actually receiving any voltage. the only way to know which of the two problems exists is to open the device and test voltage going to the battery through the charging port and then the voltage across the battery itself. you can find a teardown here:
[guide|114932]
you would need a multimeter to test the voltages. you can find how to do so here:
[guide|5539]
this will work for any battery.
hope this helps, report back for any further assistance if needed.