Sony Xperia Z Tablet SGP311 won't start after failed Lollipop update

Question

I cannot get my Sony Xperia Z table SGP311 to start after I allowed it to update with Android Lollipop.
Every time I have the device over the past week I was getting notifications to update. I finally accepted. (I wish I hadn't).
After the update notified me that it was unsuccessfil, I have the following situation:
If I plug in the charger, the indicator light will come on red, and screen will briefly display a charging icon. If I then unplug charger, the red light will go off.
From an 'off' state, charger not plugged in, if I press the Power button until the indicator light blinks blue once, the screen will appear with:
SONY
XPERIA
and then goes back to a blank screen. It will repeat this every 30 seconds.
If I turn off the device by holding Vol+ and the Power button simultaneously and wait for the indicator light to flash 3 times, charger /PC not plugged in (Let me call this state "OFF" for short), and then:
1. From OFF, then try turning on with Vol+ and Power button simultaneously: nothing will happen.
2. From OFF, go to PC Companion on my desktop PC, Support Zone > Phone/Tablet Software Update > Connect: nothing will happen.
3. From OFF, go to PC Companion > Support zone  > Phone/Tablet Software Update > Repair my phone/tablet > Another phone/tablet > select SGP311 > hold Vol- while inserting USB cable from PC, blue inicator light will come on, update process will begin, then terminate with "device update error", so "Retry', but this terminates with "download error", and  "check your internet connection". But there is nothing wrong with my internet connection!
Any more ideas?
Solved!
Go to Solution.

Answer 1

Thanks for your contributions.
After Pressing Power+Vol-up for 10 secs (during which indicator light gives 1 blue flash then a 5sec pause, then 3 blue flashes in quick succession):
I attempt the PCC repair, by pressing the vol-down (the side of the Volume rocker switch furthest from the Power button) as I simultaneously plug in the cable from the USB port.
When I do this, I see the indicator light flash red momentarily (<0.5sec) and immediately momentarily (<0.5sec) flash green. 
The updater begins but always aborts about 1 minute later with an 'Error 100' message.
I have no other internet connection but I supose the next stage of all this is:
1. Learn all the indicator light colours and flashing sequences so I know what the device is messaging. Is there a code guide I can download?
2. Install PC companion on my laptop and try using that.
Also, how do I know if the device is charging from a wall charger? Will it charge when the device is power-off? Should I see an indicator light, and if so would it be always-on, or only flash or remain on for a certain time after plugging in the charger?
I should never have given in to the nag messages to update the OS. What a headache.