OK, all done - thank you again, i really can't thank you enough. Its great to have my cloud back and I have put quite a lot of work into it lately.
Just to torture you a little more though, I now have this on the overview screen.
It's important for the security and performance of your instance that everything is configured correctly. To help you with that we are doing some automatic checks. Please see the linked documentation for more information.
There are some errors regarding your setup.
- Last background job execution ran 1 hour ago. Something seems wrong. Check the background job settings
- The database is missing some indexes. Due to the fact that adding indexes on big tables could take some time they were not added automatically. By running "occ db:add-missing-indices" those missing indexes could be added manually while the instance keeps running. Once the indexes are added queries to those tables are usually much faster.
- Missing index "cards_abiduri" in table "oc_cards".
Please double check the installation guides ↗, and check for any errors or warnings in the log.
The log says this quite a lot:
Info: Deprecated event type for \OC\Repair::step: Symfony\Component\EventDispatcher\GenericEvent is used
at 2021-02-07T16:20:08+00:00
The database problem has been there from the start, so I should address that now. Is this just a case of running occ db:add-missing-indices from the terminal?
The bit in red about the background job popped up after the update, but I've just been back to nextcloud, messed about, gone back to the overview page and the message has vanished. No need to worry about that anymore then. 

I have also run a security scan and I am getting an 'A'
The only thing it has a warning about is:
___Host-Prefix
The __Host prefix mitigates cookie injection vulnerabilities within potential third-party software sharing the same second level domain. It is an additional hardening on top of 'normal' same-site cookies.
Can I fix that?
Lastly, and so that I never have to bother you about upgrading again, what do you think I should do? I am thinking we gui upgrade and if that fails, do it by the command line as you have shown me today? Safe, do you think?
A gazillion thankyous,
Nick.