g8441 red and green lights on dead screen

[Xeno-man]

My ancient Xperia XZ1 G8441 has given such excellent service I am really reluctant to bin it, not just yet anyway. 

I need to get it back to copy over the stuff I didn't back up (most of it is backed up but some stuff is not, e.g. phone numbers). 

After a few hitches, I seem to have hit a big problem. The battery ran down almost to nothing before I attached the charger. It flashed the Sony logo for two seconds and switched off, red charging light came on, it was dark for about five seconds, and then the Sony logo appeared for two seconds, and then went off. It went on like this indefinitely so long as it was connected to the charger. 

When not connected to the charger, nothing. I tried holding down all the buttons in every conceivable combination and can only get the occasional vibrate, no activation. It is now on my desk connected to the desktop iMac and is flashing two seconds red, and two seconds green. A few minutes ago a low-battery symbol appeared, and then went. After another session pushing all the buttons, I got a blue light briefly. Now it's back to alternate red and green lights. 

I love this phone so much I would get another in a shot if I could, even though I know it won't take the latest Android. Can anyone help me get it back to life, if only temporarily so I can download the rest of the unbacked up data?

Thanks in advance,

[Xeno-man]

I pressed the side buttons again and now I have a continuous blue light instead of the previous alternating red and green lights.  I know this means it is charging, but it has been on for over an hour and still won't start.  

[Xeno-man]

If I press all side buttons at the same time, after about a minute, the continuous blue light reverts to alternating green and red lights. If I press the side buttons again, after that, the light goes back to blue again. Mystery!