@ANDREW_HAMMETT I am sorry this is happening to you and I can tell you that something like this happened to me with the C5 Ultra and I know I've been able to recover full functionality. Let me tell you that it may be an issue of battery calibration as well some software disrupting regulat functionality.
1st test I'll ask to perform is to remove SIM cards and SD cards, leave the device off and try to recharge the device while off. Once the phone is fully charged, try to turn it on and check if it's working properly without SIM or SD card.
If it works fine, turn off and insert the SIM card, turn back on the phone and check if the phone works properly. If it doesn't beahve correctly you know the SIM card is at fault, you may want to replace it with a new one by contacting your carrier/provider.
Then try with the SD card, if with the phone battery life is affected then you know the SD card is at fault, you may want to replace it.
If with or without SD/SIM the phone recharge correctly when the phone is off and it misbehave when the phone is on then you may want to perform a software repair using Xperia Companion.
Let me know how it goes.
