I’ve been told Neon has a somewhat different grub setup from Kubuntu etc so I shouldn’t just edit /etc/default/grub (certainly what’s currently in there doesn’t match the grub menu I see). Can anyone advise where I do edit it? I want to reduce the timeout on the OS selection.

I'm in Neon for years, totally reinstalled some time ago and I have edited /etc/default/grub succesfully.
In your case
GRUB_TIMEOUT=5
And...
# If you change this file, run 'update-grub' afterwards to update
I've used Grub Customizer too but actually results have been better editing directly that file.
Of course, I'd suggest doing a back-up copy of the original file first.
Regards,

It is the theming that makes Neon slightly different from Ubuntu. The standard configs are edited as usual.

But look at some of the files Ubuntu has in /etc/grub.d.
There is one in there, 30-os-prober that actually sets the timeout to 10 if it is set to 0 in the grub config, in some situations.

More info and ways to fix:

https://www.py4u.net/discuss/1131653
https://askubuntu.com/questions/1036091 ... untu-18-04
https://bugs.launchpad.net/ubuntu/+sour ... ug/1273764

Thanks guys, I’ll try that, and read those links.

The graphical tool to write your grub configuration is grub-costumizer.

Just take care, reinstall grub2 works just manually with konsole

type update-grub2 into konsole to update manually