Hi Hydrurga - many thanks for your reply. Since posting I have had two breakthroughs.
1.
https://sites.google.com/site/easyli...ject/reserve-8
which gives Mint 18.3 specific guidelines. and
2. Click on the icon on the tray.
Open the "Software Sources" program.
Click on "Additional repositories".
Uncheck the entry for "CD-ROM Installation Disc".
Click on "Update the cache" to update the cache and confirm that the entry was removed.
This was the only issue remaining after the previous instructions I posted.
I hope this proves useful for others with the same issue.