Well … resolved now and it appears to have been a caching issue.
Despite trying to clear cache and re-inistalling the plugin (several times) it appears that one or more of the plugin files were not re-loaded. After I manually edited class-moowoodle-testconnection.php (to add another log record) the Test Connection facility started working so I am assuming that the file change forced a full/proper cache refresh. Something other developers might want to be aware of.