Just an update. I removed all power and held the power button down for 20 secs. On trying again the computer went a little wild with the screen flashing and then attempted to display the boot screen, (windows did not shut down correctly etc) but it was fading in and out from the bottom lh corner, sort of blooming if you know what I mean. It seemed to try several times to boot, but had now returned to the black screen.
Google Pixel 3 review