Getting a 404 error in backend – cannot access settings

After initial install it worked fine. Then I had cause to clear the browser cache and delete cookies.

First I lost access to the settings page getting a 404 error. Later I lost all access.
Have removed the plugin and re-installed a couple of times now but no joy. Even a crude manual deletion of the plugin directory doesn’t work.
Have purged cache’s – no change.
No other issues with the site’s backend as far as I am aware.
It looks like some settings or files are retained after deletion but no idea what or where these might be.

Have installed the plugin on another site I manage and all is fine.