It had done nothing (with Pass 1 progress stuck at 25.3%) for around 10 minutes, but as you suggested I have re-run Fix-Disk and left it even longer. This time, it eventually resumed, and completed. The "read-only" issue has been resolved, and now the web browser is working. It remains to be seen whether the intermittent freezing when accessing the Media menu has also been resolved, but I can now press ahead.
Thank you both very much for all your help and support.