Hi again, @duncan
Hmmm... I suggest that you start by running the following command:
sudo apt-get update --fix-missing
... and then run (again) the following command (that you also executed):
sudo apt-get install -f
Please, reply again to tell us how that worked.
Those two commands are recommended in the "Fixing Broken Packages in Ubuntu/Mint/Debian" section of the following article / web page:
You may also want to read the following discussion here in the Ubuntu MATE Community, started in May 2021, by @mack87-dash (Sekhar Suman Dash):
... and, in particular, in that discussion, the following reply by the late "mdooley" /
Michael Dooley (who would unfortunately pass away in November 2021): How to fix "Software index is broken"? - #24 by mdooley where he recommended reading the following two articles:
Still, in that discussion @mack87-dash replied to Michael Dooley to report that running the command sudo apt-get update --fix-missing solved the issue:
I hope this helps 