OK,
I have finally given in. I've tried all the advice in this thread (thanks for all your efforts!) and still have the same issue. This device is fundamentally flawed. I lose 8-10% of battery life overnight after a full shutdown. Not modern standby. Not sleep. Not hibernate.
I've reset the device. Have installed all the latest firmware. Nothing else installed except drivers. It's just a poor design and Microsoft have not once offered a solution. Now I'm out of warranty. Absolute dogs.