XPS 13 9300 Wake from sleep

Still fixing my ultrabook 'kit' from Dell. Next on the list is its inability to wake from sleep. I get as far as the Dell logo and then it hangs. If I press the space bar after a bit the Windows Hello facial recognition leds flash and the logo turns into white/grey fuzz. The only way out is a hard restart. I have reflashed the BIOS.

Any ideas anyone?

Thanks in advance

EDIT: after the Windows Hello LEDs flash, the external monitor starts normally and just the laptop screen hangs with the white noise square, so the laptop is starting, but the screen is not.