Thank you all!

I could do with a graphical keyboard or character map to type the dead letters until I have the new keyboard.

After using the passwd -R command
I followed your excellent step-by-step description, exited the chrooted system and unmounted /mnt.

(Without the exit it didn't work.)

Apparently passwd has built-in chroot

# passwd -R /mnt

You're a genius !!!!!!

Just copy all the required text from an existing document or website in to a text file and delete what you need until you have your password in the file.

su  select the text and middle click in the terminal press enter not return

edit: Then change your password :-)

Sorry if I'm being dense. I can only get into single user mode.

You mean su into root or my personal user from

I don't have a file containing the word but I could find a b somewhere.

su  select the text and middle click in the terminal press enter not return

edit: Then change your password :-)

What's wrong with copying and pasting your password in?

How? Where do I copy it from?

The ultimate goal is to change my personal user name and password, both of which contain b and n, the dead letters.

Now bind the directories:

# mount -o bind /sys /mnt/sys
# mount -t proc proc /mnt/proc
# mount -o bind /dev /mnt/dev
# mount -o bind /dev/pts /mnt/dev/pts

.

I probably need some of this.
But how do I type bind?

chroot /mnt

Error:

/bin/sh: 0: can't access tty: job control turned off

Once booted into single user mode, I executed

mount /dev/sda2 /mnt

No more read-only warning!
Mounted with write access.

Can I run firefox in single user mode?

Or can I just do command line stuff?

Literally, that's all I want in the short term until I have a new keyboard.

You might need to run /sbin/fsck -y /dev/sda2 or /sbin/fsck.ext4 -y /dev/sda2 to mount it rw.

I tried this. Came back as clean.

Then

mount -o rw /dev/sda2 /mnt

Warning: source write-protected, mounted read-only

Any more ideas to mount read write?

What about

mount  -o remount,rw /

I can use wildcards for the mount command, but how do I type remount without n ?