Right. My animated icon has reappeared in the System Tray.
Firstly, I did an Avast "repair install" via Win Control Panel.
Then I had a look at the Settings - Troubleshooting and compared them with my operational iconised laptop.
Starting from the top (Enable Rootkit) as Item 1), and ending with "Enable Hardware-assisted virtualisation" as 8, my settings are now:-
1. On (i.e. checked)
2. On
3. Off
4. On
5. Off
6. On
7. On
8. On
Previously, I had 5) "Load Avast services after.." etc. checked because of a previous problem (now sorted via .ini file). Item 6) "Do not use Chrome..."etc. was previously unchecked, so I have checked it.
I can see that delayed starting Avast services might not help, but not Chrome. As it's working, I don't propose to experiment further to see if I can narrow anything down; at least not for the moment.
I really don't like it when things fail or work and I don't know why!