sudo grub-reboot '1>2' && sudo reboot
Grub counts from 0
1 is the second line of the Grub menu i.e. Advanced Options for Ubuntu.
2 is the third option from the second line i.e the previous kernel.
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Thank you so much! It worked a treat.
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Splendid - Pleased to have helped
I also changed the topic title unusable > unreliable
By the way, did your USB2.0 hub arrive?
Any success with typing within Grub?
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The USB 2.0 Hub made no difference but it sounded like a good idea. So i think that must rule out Legacy USB if it were to be an option on my motherboard. At least now I have a workaround. Perhaps someone in the future will figure it out.
I have fixed it!
The GRUB display defaulted to 3840x2160.
I entered GRUB command shell and input:
set gfxmode=1024x768
terminal_output console
normal
and GRUB suddenly worked!
added this line to my GRUB
GRUB_GFXMODE=1024x768
then
sudo update-grub
Rebooted and I get a fully functioning GRUB menu albeit in 1024x768!
I did try 1920x1080 but it didn’t work.
4K must be too much for the display driver to handle at that point in the booting process.
It might also explain why install media worked as they often run at lower default resolutions.
I realise that I should have mentioned what display I was using but I didn’t think it was relevant until now.
The tip off was from MS Copilot AI, the same one that told me to update my BIOS to get legacy USB options that never existed on my motherboard, and told me to buy a USB2.0 Hub (fortunately is was only £4).
In any case its been an interesting couple of days
Thanks all 
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