Solved by @apljdi in another thread - ta muchly.
Just in case any other non-techy person finds this (like I tried to do, when searching past threads trying to solve this issue) my issue was that the permissions on my .htaccess file (in the very top folder of my site) were wrong. WordPress told me this every time I tried to change my permalink settings, but the message was at the bottom of the page so I missed it.
I used my FTP client to download the file, opened it in Notepad, pasted the code given to me by WordPress into it (overwriting everything that was there previously) and re-uploaded it. Fixed.
Thanks for your excellent help, @apljdi.