HTC One M9 - Stock, never modified. Bricked after OTA update 15/06/2017 Please note that:
This phone has never been reprogrammed/rooted/changed in any way/anything else like that. It is as stock as the day I bought it 2 years ago. I am happy to do whatever it takes to resolve the issue below.

The phone can be turned on, but will only do one of the following things:

-Black screen, old-style htc logo in centre, short vibrate, turn off, endlessly loop.

-White screen, green htc logo with "powered by android" text at bottom, short vibrate, turn off, endless loop.

Holding volume up+down together gets me to the boot screen with the following:

* Software status: Modified 
 LOCKED 
 S-ON *

 to previous item
 to next item
 to select item

REBOOT
REBOOT TO BOOTLOADER
BOOT TO DOWNLOAD MODE
BOOT TO RECOVERY MODE
POWER DOWN

When I plug in the phone to my PC, the USB notification sound plays, but only if the menu above is showing.

Selecting any of the options reboots the phone and goes back to the menu above, with the exception of the "BOOT TO RECOVERY MODE" option. Selecting that takes me to a black screen showing the old htc logo in the centre and:

Failed to boot to recovery mode
Press volume up or down to back to menu

This build is for development purposes only. Do not distribute... etc.

Pressing vol up/down at this point gives me the first mentioned menu screen, but below the * S-ON  line is  Security Warning *.

So, my question is:

1) Can I get my phone out of this loop and back to booting normally?
HTC have sent me this, but as far as I can tell it's useless in our case:

(Posted as answer, not comment, because of size and formatting)

(Not sure what they've done with the numbering)


  factory reset the phone in order to get out of the  boot loader page, please follow the below steps
  
  3.Press  and hold the Volume Down key.
  
  4.Turn the phone on by pressing the Power key until the phone vibrates, then release Power.
  5.Continue holding the Volume Down key.
  
  6.Release the Volume Down button when the black screen with red and blue lines of text appears.
  
  -
  
  1.Press  the Volume Down button repeatedly until reboot to bootloader is highlighted, then select it with thePower button.
  
  2.The device will display a white screen with colored text.
  
  -
  
  1.Press  the Volume Down button repeatedly until BOOT TO RECOVERY MODE is highlighted, then select it with the Power button.
  
  2.The screen will display the white HTC screen, and then turn black.  After several seconds, a picture of the phone with a red triangle and exclamation mark will appear
  
  -
  
  1.Hold the Power button, while pressing and releasing the Volume up button.
  Note: This can be done at any time after the screen turns black.  If this button combination is pressed before the image of the phone with the red triangle appears, it may take several seconds for the recovery menu to appear at the top of the screen.
  
  2.Another black screen with red and blue text will appear.
  
  3.Press  the Volume Down button repeatedly until wipe data/factory reset is highlighted, then select it with thePower button.
  
  -
  
  1.Press  the Volume Down button repeatedly until Yes – delete all user data is highlighted, then select it with thePower button.
  
  2.Once the data wipe is complete, make sure reboot system now is selected and press the Power button.