@mrt10001 Success! I tried what you suggested but I had no luck. I tried what the twrp site said and again no luck. I flashed Android v9 and started again. And that worked. Thanks so much for your help, MrT; it got me started!

@mrt10001 OK! I got to fastboot mode, then turned phone off: work for tomorrow! as it's 8:30pm in UK now.

@mrt10001 Thanks. I tried that but no good.
The screen displayed says 'Your device is corrupt and can't be trusted and may not work properly'. I pressed for 10 seconds and screen went off but came on again of its own accord. I turned it off and pressed on before it came on, and back to the same screen.
So I can't seem to get to fastboot screen.

This is a compilation from github 'Pixel 3a XL unable to enter the Ubuntu system #2759'

Chanshunli posted:
Unable to enter the Ubuntu system, ubports installer directly entered UBports Recorvery after installation.
There followed a picture of UBports recovery screen, with logo etc in Yellow. [Too big for inclusion.]

I added:
Using installer 0.9.6-beta:
/tmp/last_kmsg has, at end of ile, many 'cannot get SElinux label on /dev/block/mcblk... device no data available'.
But the colour was orange, not yellow; and Slot b
It seems the phone cannot now be switched off. From the screen with the message 'Your device is corrupt. etc' pressing vol down and/or power results in screen going blank for about 1 second, but then the message screen reappears.

I tried the solution to post: 'Stuck on Google logo instead of recovery', but no effect (older version after all).