Well, after much messing around (days of it - good job I'm retired) I managed to sort out this mess.

Firstly, I'm seriously underwhelmed by the help I received from this forum. I've only asked 3 questions (to deal with specific problems) and have only received limited help with one of them. Ubuntu will only remain a niche system unless that changes. A serious number of users appear to have audio problems similar to mine and few get any answers via this forum!

Eventually, after many attempts at fixing the problem, I reloaded 16.04 (rather than 18.04) in dev/sda8 and the audio problem went away. An issue I hadn't reported was that the partition I was using for home (dev/sda7) was formatted in ntfs and I suspected there was a fundamental permissions issue around that. I eventually came across a thread about using ntfs with Ubuntu and managed to get the system to work, though not perfectly and it seemed that all directories had acquired root permissions. Eventually, I bit the bullet, backed up my home directory (and also copied it to my Win10 partition). I then loaded a clean version of 18.04 to dev/sda8, reformatted dev/sda7 as ext4 and then set up dev/sda7 as home and copied the new 'clean' home to it. I then carefully selected bits of my old home from within my Win10 partition and pulled them across to dev/sda7. That way, I was able to restore all my original files and important settings such as my Firefox bookmarks and history and my Thunderbird email accounts, addresses, saved mail, etc. Permissions for all these files now show my username, rather than root.

I suspect this answer might not directly help anyone, but might give some clues to those with similar problems. It looks like my gut instinct of permissions problems was correct and the results seem to bear this out. Unfortunately, this issue was spread across 3 separate problem areas - audio, hdd file systems and permissions - something I didn't realise on my initial posting,