Greetings to all !
Further to my last post I submitted this question to the linux Mint forum. I got a couple of answers back but neither directly solved the problem. However, they lead to further experiments and I have now found the cause.
I run my browser and email client (Thunderbird) in a sandbox, using the "firejail" utility. If the condition of default browser is set whilst the application is running sandboxed then, the setting will be lost on exit because firejail stops anything being written outside the sandbox, except to the directory "Downloads".
If Opera is run outside the sandbox then the condition "default browser" can be set and will persist after exit. However, Opera running in a sandbox is (functionally) a different entity so, one cannot set the default condition and hope it will be preserved inside the sandbox. The solution to this probably requires a different custom profile for Opera and I will probably develop this at some future date.
I hope this report will be useful in some way to others.
Best regards to all ---