I did the sequence of removing, cleaning and then installing ubuntu-pro-client and it worked, in that now there are no key errors when I do an apt update (other than the OpenFOAM ones which were always there).
However I then went on to try removing, cleaning and then installing again some other packages and then completely lost my desktop! I can still log in and have networking on that machine, although it came up at the top as KDE-Neon! So I tried sudo apt install ubuntu-deskopand had errors about unmet dependencies due to version numbers which I think might refer to KDE packages. So I went through each one removing, cleaning and installing again, and hooray, got ubuntu-desktop back! Also now I can install update-manager and all of the other packages I was having problems with and none of the weird version numbers, so it seems fixed.
I’d still suggest to take a re-install into consideration (and do a proper backup beforehand indeed) once 26.04 is out next month since you will not really know what other low-level packages have been replaced or are broken …
Although, it is still labelling the root as KDE neon (the other two partitions are for Windows which I have a dual boot). My guess is that at some point I added a KDE repo by mistake and some packages from there got installed at some point when I did an update and replaced the standard Ubuntu ones and then the havoc set in.
Absolutely! Thank you for your help. Yes, I was thinking about doing a full wipe and re-install when 26.04 comes out, as I knew this laptop was turning into a bit of a Jenga tower! Now I definitely will do that, but hopefully this will now allow me to keep using it until then!
