Xperia Z5 Premium doesn't power on

I dropped my Z5 Premium Dual, it immediately turned off and I’ve not been able to power it on since. The screen doesn’t look damaged, in fact it was only a week old. The phone was in a case at the time.

Desperate to get my data off the internal memory. I was hoping I could just switch the motherboard out and put it into another Z5 premium. So far no luck, here is what I have tried.

I have another Xperia Z5 Premium, this one is UK so its single SIM rather than Dual, but effectively its the same phone. I thought I’d use this a donor.

1. removed the battery from my Z5 Premium dual and tested it works ok in the Z5 Premium (single). Yes Battery works.
2. removed the power button assembly, with ribbon, vibrator module, etc from my Z5 Premium dual and confirmed it works in the Z5 Premium (single). Yes it works.
3. removed the mother board from my Z5 Premium Dual and put it into the Z5 Premium (Single). No the Z5 Premium (single) will not power on.

Test 3 implies the motherboard from my Z5 Premium Dual is not working. However, when I then took the mother board from the Z5 Premium (single) and put it into the body of the Z5 Premium Dual, the Z5 Premium Dual will still not power on. So I’m not 100% sure. Perhaps there are two faults on the Z5 Premium Dual; mother board and screen for example. Though it does seem unlikely to have two sub-assemblies fail.

Any suggestions on how I might get my personal data off the Z5 Premium Dual?

And before anyone asks about backups, some of the data was backed up to the SD card and is safe, but not all (otherwise I wouldn’t be asking).