Tesco Hudl 1 - blind factory reset

I have this Tesco Hudl 1 which a friend literally found under a hedge, and gave me to see if I could fix it. It takes charge and if you try to power it up there are signs of life (faint screen backlight glow) but no display. However if you plug in an external HDMI screen it produces a display, and even responds to a mouse and keyboard plugged into the USB port. But it boots to a lock screen and I've no idea what the password might be.

I tried a replacement screen but it was no better. Presumably the problem is with the LCD driver. However, it'd be neat and an interesting challenge to get it working with the external screen, mouse and keyboard and possibly to root it.

It should be possible to do a factory reset by pressing the power and volume-up buttons, which should show the system recovery menu, but unfortunately this menu is not displayed on an external screen, and I don't know how many times to press Volume Down to get to the factory reset menu item.

Could someone please tell me the system recovery menu structure and the precise sequence of button presses to do a blind factory reset, including any delays necessary after releasing the power and volume-up buttons before the system recovery menu appears.