HTC One M9 - Stock, never modified. Bricked after OTA update 15/06/2017
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.