Resolved
Hello Ray
Well, perserverance has paid off.
Using the Adobe Creative Cloud Desktop, I uninstalled the Adobe server behaviours then reinstalled that extension.
When i started Dreamweaver, the 'stuck' WA MySQLi extension completed its uninstall.
The MySQLi extension reinstall using ExManCmd failed again, but using the DMXZone extension manager all went well.
No idea what happened but all is now good.
Thanks as always for your prompt support.
Steve