Z5 Premium will not power up. All reset codes tried

I have had a Sony Xperia Z5 Premium come in that turned off while partially charged and now will not respond to any input.

No LED lights, no vibration and no connection to PC or PC suite.

I have tried every hard reset button combination and nothing seems to work. The handset is in excellent condition with no outer damage. I assume the device is 18-24 months old.

The battery was charging and discharging normally before it died.

Is this a dead batttery or something more sinister?