Well David, I have eventually found resolution!
Knowing how frustrating it is to research resolution for a problem on the net, only to find many posts have gone 'quiet' on a subject, suggesting a solution has been found, but not shared, I will offer the following advice for anyone else who has experienced a similar problem to mine...
After trawling through many posts all over the internet, I found that the most likely cause for the loss of extensions for the Avast Secure Browser (even after uninstalling and re-installing), was due to remaining corrupted Avast files left on the computer after uninstalling. After many failed attempts to delete these, even going to the extent of creating a second administrator user on my computer (which also didn't work), I eventually found that Avast do have a little publicised 'Uninstall Utility' program -
https://www.avast.com/en-gb/uninstall-utility , which in my case after following the instructions, successfully deleted all remaining Avast files, so a 'clean' re-install could be implemented, thus finally reinstating the 'Adblock' and 'Bank Mode' extensions for my Avast Browser!